The present invention includes an integrated fuel processor subsystem incorporating a thermal combustor, a catalytic combustor, a quasi-autothermal reactor (QATR) and a air-fuel-steam (AFS) mixer to provide a range of operating modes exhibiting performance between that of a pure steam reformer and a pure autothermal reformer to increase the flexibility of the fuel processor to handle transient system demands such as cold starts, suppress emissions and carbon formation and improve efficiency.