NOVELTY - Removing carbon monoxide from a hydrogen-rich gas stream produced in a reactor, involves passing the hydrogen-rich gas stream through a vessel (e.g. rotating pressure swing adsorber) containing a carbon monoxide adsorbent.    USE - For removing carbon monoxide from hydrogen-rich gas stream produced in a reactor, and improving the quality of hydrogen-containing gas stream; useful in a fuel cell system having a fuel cell stack (claimed) for hydrogen purification for automotive fuel cell applications.    ADVANTAGE - The method eliminates the use of preferential oxidizer (PROX) for removal of carbon monoxide from hydrogen-rich gas stream, by using carbon monoxide adsorbent. Hence, the method cleans up carbon monoxide without hydrogen consumption and without the concomitant, undesirable excess low grade heat generation. The fuel cell system using the method, has the ability to remove carbon monoxide at startup, thus reducing the time required to produce stack grade hydrogen; has the potential to produce 99.9 % hydrogen; has the potential to recover carbon monoxide for improved system efficiency; provides elimination of low temperature water gas shift; provides elimination of precious metal catalysts used in preferential oxidation (PROX); eliminates dilution of hydrogen product with nitrogen from air used for preferential carbon monoxide oxidation; provides elimination of air compression for preferential oxidation, resulting in power savings; provides elimination of low-grade excess heat generated by preferential oxidation; and has the ability to control the adsorption unit's cycling rate, to optimize efficiency over a wide range of operating loads. The method reduces start-up duration and improves overall fuel processor efficiency during normal operation.    DETAILED DESCRIPTION - The method further involves pressurizing the vessel before passing the hydrogen-rich gas stream through the vessel; depressurizing the vessel after passing the hydrogen-rich gas stream through the vessel; purging the vessel with a gas having a low carbon monoxide concentration; and passing the hydrogen-rich gas stream through a water gas shift reactor disposed between the existing reactor and the vessel. The water gas shift reactor contains an adsorbent, which adsorbs carbon monoxide at low temperatures and desorbs carbon monoxide at high temperatures.