After my research, I am not convinced that Penal Substitutionary Atonement is a formally condemned heresy. However, it seems to contradict the sensus fidelium, or the "sense of the faithful." The Church has largely rejected it, and the doctrine seems to entail metaphysical claims which are incompatible with Catholic theology.
First, a word about the protestant formulation of the doctrine. Using this question as a guide, the early protestant formulations don't appear to make very controversial metaphysical claims. They stick to formulations that could be interpreted in orthodox or heterodox ways. For instance, from Luther:
Also they teach that men cannot be justified before God by their own strength, merits, or works, but are freely justified for Christ's sake, through faith, when they believe that they are received into favor, and that their sins are forgiven for Christ's sake, who, by His death, has made satisfaction for our sins.
And, from Calvin:
He offered in sacrifice the flesh which he took from us, that by expiation wought he might destroy our guilt and appease the Father's just anger.
The Catholic Catechism also uses language such as this. CCC 615 states the following:
By his obedience unto death, Jesus accomplished the substitution of the suffering Servant, who "makes himself an offering for sin", when "he bore the sin of many", and who "shall make many to be accounted righteous", for "he shall bear their iniquities". Jesus atoned for our faults and made satisfaction for our sins to the Father.
It is important to point out that the heading for this paragraph implies that Jesus made satisfaction by a kind of substitution, but that substitution was His obedience for our disobedience, rather than His substituting His suffering for our just desserts. The Catechism does not here support the notion of Penal Substitutionary atonement as commonly formulated today, where Christ became the object of wrath to be satisfied. Rather, His obedience is received by the Father as an acceptable and pleasing sacrifice on our behalf.
Without even an archived version of the link in the original question, I can only speculate about what the original commentor had in mind when he said that Penal Substitutionary Atonement is doctrinal heresy, but if he had in mind the sorts of things contained in these articles from Reformed websites that I found, he may be railing against the metaphysical implications, and the problems they present for Trinitarian orthodoxy.
One Reformed website states
In penal substitution, the penalty that is due to us for our transgression is paid by a substitute, namely, Jesus Christ.
Another defines Penal Substitution thus:
The penal substitutionary view of the atonement holds that the most fundamental event of the atonement is that Jesus Christ took the full punishment that we deserved for our sins as a substitute in our place, and that all other benefits or results of the atonement find their anchor in this truth.
According to Catholics, and, I believe, the Reformed tradition as well, the "full punishment that we deserve" for our sins is eternal separation from God, which is the Catholic definition of hell. "The chief punishment of hell is eternal separation from God, in whom alone man can possess the life and happiness for which he was created and for which he longs." (CCC 1035) This means that Jesus, though He is fully God and fully man, would have to somehow be separated from God in order to endure the full penalty of our sins. Not only this, but He would need to endure eternal separation... from Himself. That said, I don't believe that Reformed theologians would consider His punishment eternal (which is strange and seems incoherent if they believe hell is eternal for the damned), so let's assume He is only temporarily separated. So, either the Son is separated from His human nature, which is Christological heresy (He ceases to be one Divine Person with two natures), or the Son is separated from the Father, which is Trinitarian heresy (they must share the same, singular Divine Essence).
Now, in order to avoid this pitfall, the Reformed theologian may say that Penal Substitution was only made for the positive suffering due to man, and not for the eternal separation form God which the damned receive. In this case, he would need to explain how satisfaction is made for that deserved eternal punishment, and then some other form of satisfaction would be chiefly at play besides Penal Substitution. Otherwise, he might say that eternal suffering is due to sinners, but not separation from God. He might merely say that this eternal suffering has the effect of separating us from God. This, of course, gets the Catholic understanding of hell precisely backwards, and thus would be understandably condemned by any practicing Catholic. In the Catholic view, the eternal punishment for sin just is eternal separation from God. That is what hell chiefly is.
THE QUESTION OF JUSTICE
Alternatively, the individual could have been railing against the Reformed doctrine on account of its incompatibility with Catholic - or common sense, for that matter - notions of justice. A Catholic Answers article (worth reading in full) quotes the Reformed Theologian R.C. Sproul, saying
The most violent expression of God’s wrath and justice is seen in the Cross. If ever a person had room to complain of injustice, it was Jesus. He was the only innocent man ever to be punished by God. If we stagger at the wrath of God, let us stagger at the Cross. Here is where our astonishment should be focused. If we have cause for moral outrage, let it be directed at Golgotha.
The article then pithily responds to this, saying "So according to Sproul and others, God is good and just because He violently and wrathfully punished His innocent Son."
It is evident that a truly just God would not remove the guilt from a guilty party because an innocent party was punished instead. A good judge would not permit an innocent party to go to jail on behalf of a dangerous murderer. He would not be doing his job as judge. The murderer deserves jail, and the innocent man does not. If the judge says that he will punish the innocent man who willingly takes on the murderer's sentence because the murderer has repented, then he still sins against justice. Rather than this, if the judge is convinced of the murderer's repentance, enough to let him go free, then he should simply let him go free.
In fact, the sacrifice of Christ on the cross was not strictly speaking necessary for atonement. That is, God could have simply forgiven without a sacrifice. But, He willed what was most fitting in His incarnation, and in His suffering and dying on our behalf, and His death was necessary for our atonement insofar as God willed our atonement by means of His own death. (See ST III Q46 A1)
Penal Substitutionary Atonement, at least in some formulations, contradicts established Christological or Trinitarian dogmas, and also appears to contradict typically Catholic notions of justice.