depending which zombie theory

In zombieland they're only infected with a mad cow disease, so technically they aren't really "dead" and would still produce body heat. But even other types of zombies would need body heat or the body wouldn't work. Our body heat makes sure that the different enzymes and chemicals in our body work properly. Most wouldn't work at lower temperatures.

well when people become zombies their digestive system stops and the only heat i can think of would be blood and decompasion in the tummy area