A reaction vessel that integrates and balances an endothermic process with at least one exothermic process of the fuel cell system. Preferably the exothermic process is conducted in stages to provide more uniform and/or controllable heat generation and exchange, and to produce a uniform and/or controllable temperature profile in the endothermic reaction process. The invention allows for the elimination of the working fluid loop of prior art systems that had unsatisfactory response times at startup, and during transient conditions, and also added to the overall mass and volume of the fuel cell system.