What is Adiabatic flame temperature?
The temperature combustion gas would reach if none of the heat released escaped to the surroundings.
It is the upper limit used to judge material temperatures and \mathrm{NO}_x tendency. Real flames lose heat, so they stay below it.
Burning methane in air at the stoichiometric ratio (\varphi = 1), starting from room temperature and atmospheric pressure, gives about 2220 K.
Choose HP in the Chemical Equilibrium calculator, or use an Equilibrium Reactor in HP mode in Chemical CAD. The resulting temperature is the adiabatic flame temperature.
At constant pressure with no heat exchange, enthalpy is the same before and after reaction:
H_{\mathrm{reactants}}(T_0) = H_{\mathrm{products}}(T_{\mathrm{ad}})
At high temperature part of the \mathrm{CO_2} and \mathrm{H_2O} dissociates and absorbs heat, so the result is below the complete-combustion estimate. The maximum usually occurs slightly on the rich side of \varphi = 1.