Kenmore 80 series gas dyer will not heat up to dry the clothes in the dyer, but it will start.

Does the flame on the gas burner start and then quickly go out, or does the dryer tumbler start and with out the burner coming on? If the burner starts and quickly goes out it could be as simple as the vent being clogged with lint. If the vent is clogged the heat quickly backs up and shuts off the gas as it senses that it is hot enough.