There are two main ways excessive heat can build up that would cause your problem.
1. You have blockage in your vent system. You should tear down the dryer portion and clean it out, and as part of this you should remove the cover from the blower and clean it out and check to make sure the blower is turning correctly.
2. Your cycling thermostat could be staying on. I would replace it to be on the safe side.
You definitely should not operate the dryer until you have this repaired. You are looking at major fire potential. Remember to disconnect the dryer before working on it.
Good luck and let us know what you find.