Methane + Oxygen
> Heaters > Heat
If you're being pedantic, Methane + Oxygen > Heat > Heaters.
Kim Stanley Robinson uses Hydrazine for his motors in his books, along with solar power. I suppose oxidation is out, because of the scarcity of oxygen and it's use for breathing. Though, I don't ever remember him mentioning methane at all. Nitrogen is the element they are always looking for.