A catalytic combustion unit for a fuel cell system is provided. The catalytic combustion unit includes a reactor having a porous medium with a catalyst deposited thereon. The reactor is disposed adjacent a heat exchanger and adapted to receive an air stream and a hydrogen stream. The reactor is further adapted to promote an exothermic reaction and modulate a temperature of a fuel cell stack. A fuel cell system and method employing the catalytic combustion unit are also provided.