NOVELTY - A fuel processor system comprises adsorbent apparatus downstream from a first reactor (22) having a vessel housing adsorbent adapted to adsorb carbon monoxide; and methanation reactor downstream from the adsorbent apparatus. The apparatus reduces carbon monoxide in a first gas stream to produce a second hydrogen-rich gas stream containing carbon monoxide.    USE - For producing hydrogen from a liquid or gaseous fuel, e.g. hydrocarbons, e.g. gasoline, alcohols, e.g. ethanol or methanol for PEM fuel cell.    ADVANTAGE - The invented fuel processor system reduces the carbon monoxide concentration of a hydrogen-containing gas stream.    DETAILED DESCRIPTION - A fuel processor system comprises first reactor that produces a first hydrogen-rich gas stream (9) containing carbon monoxide; adsorbent apparatus downstream from the first reactor having a vessel housing adsorbent adapted to adsorb carbon monoxide; and methanation reactor downstream from the adsorbent apparatus. The apparatus reduces carbon monoxide in the first gas stream to produce a second hydrogen-rich gas stream containing carbon monoxide. The downstream components are sized to reduce the concentration of carbon monoxide to a level for use in a polymer electrolyte membrane (PEM) fuel cell stack. INDEPENDENT CLAIMS are also included for:    (A) a method for reducing the carbon monoxide concentration of a hydrogen-containing gas stream comprising generating a first hydrogen-rich gas stream containing a first carbon monoxide level; generating a second hydrogen-rich gas stream by passing the first stream through an adsorbent apparatus; and passing the second stream through a methanation reactor to produce a third stream with lower carbon monoxide content than the second stream; and    (B) a method of operating a fuel cell having anode, cathode and proton exchange membrane comprising supplying the anode with a hydrogen-containing gas stream.    DESCRIPTION OF DRAWING(S) - The figure is a schematic diagram of an autothermal reactor based fuel processor.    Stream (9)    Output stream (21)    Reactor (22)    Water vapor stream (71)    Water stream (123)