NOVELTY - Multi-stage vaporization system comprises a vaporizer comprising at least two heat exchange sections, each having at least one first channel through which the fluid stream flows and at least one second channel comprising heating mechanism, a liquid supply mechanism for adding liquid to a downstream heat exchange section of the heat exchange sections, and a reactor receiving the vaporized outlet stream for reaction within the reactor.    USE - The multi-stage vaporization system is used for fuel cell systems.    ADVANTAGE - The invention decreases the transient response time for fluid vaporization for meeting system power demands.    DETAILED DESCRIPTION - Multi-stage vaporization system comprises a vaporizer (16) having an inlet receiving a fluid stream comprising a liquid which is vaporized to form a vaporized outlet stream, and comprising at least two heat exchange sections (24, 26, 28) serially arranged in a direction of flow of the fluid stream through the vaporizer for vaporizing liquid in the fluid stream in stages; a liquid supply mechanism for adding liquid to a downstream heat exchange section of the heat exchange sections; and a reactor in fluid communication with an outlet of the vaporizer and receiving the vaporized outlet stream for reaction within the reactor to produce a reaction product. Each of the heat exchange sections comprises at least one first channel through which the fluid stream flows and at least one second channel comprising heating mechanism. The first and second channels are separate from one another and are arranged in heat transfer relationship with one another. The added liquid mixes with the fluid stream exiting an upstream heat exchange section of the two heat exchange sections for vaporization within the downstream heat exchange section.    An INDEPENDENT CLAIM is also included for a method for reducing the carbon monoxide content of a gas stream while vaporizing liquid in a fluid stream, which comprises providing a gas stream which comprises carbon monoxide, providing a fluid stream which comprises liquid, reacting a portion of the carbon monoxide with oxygen in an exothermic reaction while transferring heat from the exothermic reaction to the liquid in the fluid stream to vaporize the liquid while maintaining the fluid stream and the gas stream separate from one another, adding liquid to the vaporized fluid stream, and reacting another portion of carbon monoxide in the gas stream with oxygen in an exothermic reaction while transferring heat from the exothermic reaction to the fluid stream to vaporize the added liquid, while maintaining the fluid stream and the gas stream separate from one another.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of an exemplary fuel cell system including a multi-stage vaporization apparatus.    Vaporizer (16)    Manifold (22)    Heat exchange sections (24, 26, 28)