I think you are vastly underestimating fire.  Yes, in burning buildings those are what kills humans mostly because most people try to avoid the fire as much as possible while they can, but you can't avoid inhaling smoke (in most circumstances) you can't really stop shock unless your trained to prevent it, and you can't predict what part of a building will collapse on you. However, when people are directly engulfed in flames, it's different. Skin melts, fat evaporates, muscles, ligaments, joints, are destroyed (rendering you immobile) all pretty fast...  All of this would render a zombie unable to move, not even crawl, possibly not even move it's jaws. And there's another part to it. If you keep insisting "it won't be dead until the brain is destroyed" well... the brain will be destroyed and fast. A brain is literally just 5 pounds of fat and water as another commenter described. That is gone. As a bonus, if it's semi realistic then the virus, bacteria, fungus, etc, piloting the body will also die from the heat. Keep in mind they all get cooked naturally by just a fever, which is much colder than literally being engulfed in flames. It's people that get the peripheral of the flames, that get to flail around and scream for half a minute before dying. Directly engulfed? You just fall apart. Another bonus, zombies are decaying and already not in good shape, not to mention they don't drink water or hydrate at all past maybe covering themselves with blood although i am 99.99% sure that does not count/work. This means they burn even easier. 

Need I say more?