NOVELTY - A reactor (2) comprises a reactor inlet (8), a reaction chamber (4), a reactor outlet (10), and a microchannel heat exchanger (12), where the reaction chamber contains a zirconia-supported, alkali-metal-modified ruthenium catalyst (6) and the microchannel heat exchanger is in thermal contact with the reaction chamber. The reaction chamber has a width of less than 2 mm. The reactor comprises at least two heat exchangers that are interleaved with at least 2 reaction chambers.    USE - As a reactor in a fuel processing system and in hydrogen production system (claimed).    ADVANTAGE - The reaction of carbon monoxide with steam over an alkali-modified ruthenium-on-zirconia catalyst yields surprisingly high yields of hydrogen gas at relatively low temperatures. The use of zirconia-supported, alkali-metal-modified, ruthenium catalyst in the water gas shift reaction produces superior results. The microchannel heat exchanger enables rapid heat transfer from the reaction chamber thus allowing the catalyst to operate at near isothermal and low temperature conditions to maximize CO2 selectivity while maintaining high conversions of carbon monoxide. The reactor provides high carbon monoxide conversions, high carbon dioxide selectivity, low methane selectivity, operation at short contact times, and low temperature operation.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a hydrogen production system comprising a fuel tank; a primary conversion reactor (16) capable of converting fuel to produce carbon monoxide, where the primary conversion reactor contains a fuel inlet (18), a primary conversion reactor chamber and an exhaust outlet (20); a water gas shift reactor containing a shift reactor inlet, a reaction chamber, and a shift reactor outlet, where the shift reactor inlet is connected to the combustor exhaust outlet, and the reaction chamber contains a zirconia-supported, alkali-metal-modified, ruthenium catalyst. The hydrogen production system further comprises a fuel cell (22) to form a fuel processing system (30); a secondary CO clean up system; a fuel cell to form a fuel processing system; and a microchannel heat exchanger in thermal contact with the reaction chamber.    DESCRIPTION OF DRAWING(S) - The figure shows schematic view of a simplified fuel cell system that includes a cross-sectional view of a water gas shift reactor that includes a microchannel heat exchanger.    Reactor (2)    Reaction chamber (4)    Catalyst (6)    Reactor inlet (8)    Reactor outlet (10)    Microchannel heat exchanger (12)    Channels (14)    Primary conversion reactor (16)    Inlet of primary conversion reactor (18)    Exhaust port/outlet of primary conversion reactor (20)    Fuel cell (22)    Fuel processing system (30)