NOVELTY - An autothermal reactor for reforming a hydrocarbon fuel stream into a hydrogen-rich reformate stream, comprises a first stage catalytic reformer for partially reforming a mixed reactant stream into an effluent stream, and in fluid communication with an outlet of a premix chamber; and a second stage catalytic reformer for further reforming the effluent stream to form the reformate stream.    USE - The assembly is used for reforming a hydrocarbon fuel stream into a hydrogen-rich reformate stream (claimed). It is useful for a fuel cell system for vehicle application.    ADVANTAGE - The assembly is compact for easy integration into a vehicle application, and is capable of handling transient load requirements, particularly at start-up. It has increased efficiency through improved thermal management and inexpensive to produce.    DETAILED DESCRIPTION - An autothermal reactor for reforming a hydrocarbon fuel stream into a hydrogen-rich reformate stream, comprises:    (i) a premix chamber for mixing the hydrocarbon fuel stream with air and/or steam for producing a mixed reactant stream;    (ii) a first stage catalytic reformer for partially reforming the mixed reactant stream into an effluent stream, and in fluid communication with an outlet of the premix chamber;    (iii) a post-premix chamber for mixing the effluent stream with air, steam, and/or fuel vapor, and in fluid communication with an outlet of the first stage catalytic reformer;    (iv) a second stage catalytic reformer for further reforming the effluent stream to form the reformate stream, and in fluid communication with an outlet of the post-premix chamber and in heat exchange communication with the first stage catalytic reformer; and    (v) a thermal reactor for combusting the hydrocarbon fuel with air to form a combustion stream during a start-up period prior to supplying the hydrocarbon fuel stream and air and/or steam to the premix chamber, the thermal reactor having an outlet for supplying the combustion stream to the first and second stage catalytic reformers, for heating the first and second stage catalytic reformers to establish a normal operational temperature.    An INDEPENDENT CLAIM is also included for a method of operating an autothermal reactor for reforming a hydrocarbon fuel into a hydrogen-rich reformate stream, comprising:    (a) operating a thermal partial oxidation reactor of the autothermal reactor for a first period for thermally reforming the fuel and generating heat;    (b) heating components of the autothermal reactor during the first period;    (c) switching to a second period after the components achieve an operational temperature;    (d) premixing the hydrocarbon fuel stream with air and/or steam for producing a mixed reactant stream;    (e) reforming the mixed reactant stream in a first stage catalytic reformer of the autothermal reactor for producing an effluent stream;    (f) reforming the effluent stream in a second stage catalytic reformer of the auto-thermal reactor for producing the reformate stream; and    (g) simultaneously transferring heat between components of the autothermal reactor for maintaining operational temperatures during the second period.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of a fuel cell system including an autothermal reactor.