Freedom within limits
Freewill always exists within limits, e.g., the limitations and quirks of our minds, etc. This does not nullify freewill.
Freedom in heaven might be limited in certain ways (e.g., the ability to sin is taken away) but that is not to say it is nullified completely. People might not be free to choose evil but they are free to choose between any good and neutral actions.
People who land up in heaven – Christians – have thoroughly abdicated their desire to do evil. They spend their lives resisting, hating, eliminating sin and praying to God for this. They choose to give up the choice to sin.
Thus, in heaven, the limits of our freewill change, but there is still freewill within these limits. Freedom to go where they will, to choose what good things (or neutral things) to do, but they will never again be tempted to do evil; this is a tremendous relief to believers.
This stands to reason. Life is the test; those people in heaven have passed the test and no longer need to be tested. The test has ended completely in heaven.
The alternative does not stand to reason: It would be somewhat cruel to kick people out of the presence of God (which would be the inevitable result of sin) after them having suffered to get there, and then sent to…hell? Or some sort of purgatory? And God possibly losing believers here and there for eternity? No.
There is, in any case, less reason to sin in heaven: people will lack nothing, need nothing, want nothing. They will have God – which is, literally, everything. They will have unending joy in the presence of God; beside this everything else is meaningless.
A free world without suffering?
In light of this, it is possible to create a world with freewill and without suffering. This would be a world where no-one could choose evil (or the evil would not have bad consequences and would, then, not really be evil). If people were put straight into this world, they might seem perfectly good, when they might have been perfectly wicked if they were given the chance. These perfectly wicked people would seemingly be rewarded by God forever. (Not desirable for an all-good God.)
Instead of this, God removed us from himself to be tested in the free world with evil and suffering. Those who pass the test go on to the free world without suffering. Testing freedom, before rewarding freedom - simple no?