NOVELTY - Chemical reactor comprises reaction and catalyst-receiving zones within a core comprising at least one stack of metal plates bonded in face-to-face relationship, and three channel arrangements in at least some of the plates, to respectively deliver three reactants to the reaction zones and the catalyst-receiving zones.    USE - As chemical reactor, for effecting a chemical reaction, e.g. as a reformer used with a fuel cell, in a fuel processor with a reformer stage that incorporates the reaction zones, and for use in association with a proton exchange membrane fuel cell, in the form of conduction zones and catalyst-receiving zones (All claimed)    ADVANTAGE - The novel reactor alleviates at least some of the problems of tubular reformers (including difficulties in obtaining adhesion of catalyst to the metal substrate, difficulties in replacing the catalyst, and coupling of heat transfer and catalyst areas, which require very high activity catalyst, or over-investment in heat exchange surface). The three-stream heat exchanger permits high effectiveness to be achieved and avoids the need for a controlled split of the syngas stream to preheat the methane and water streams in separate exchangers.    DETAILED DESCRIPTION - Chemical reactor comprises a core (31) comprising at least one stack of metal plates (32) bonded in face-to-face relationship; reaction zones (27/33) within the core; catalyst receiving zones (35) within the core; a first channel arrangement in at least some of the plates for transporting a first reactant to and between the reaction zones, where portions of the first channel arrangement that interconnect the reaction zones are formed over at least a portion of their length as heat exchange channels; a second channel arrangement in at least some of the plates to deliver a second reactant to each of the reaction zones; and a third channel arrangement in at least some of the plates, for transporting a third reactant to and between the catalyst-receiving zones, where portions of the third channel arrangement that interconnect the catalyst receiving zones are formed over at least a portion of their length as heat exchange channels that are near the heat exchange channels of the first channel arrangement.    An INDEPENDENT CLAIM is also included for a method of effecting a chemical reaction in a chemical reactor, which involves directing a first reactant into and serially through the reaction zones in the chemical reactor through the first channel arrangement; directing a second reactant in parallel feeds into the reaction zones by way of the second channel arrangement, where the second reactant reacts exothermically with the first reactant in the respective reaction zones; and concurrently directing a third reactant into and serially through a catalyst contained in the catalyst receiving zones by way of the third channel arrangement and exposing the reactant to heat from the product of the exothermic reaction in its passage through the heat exchange channels of the first channel arrangement.    DESCRIPTION OF DRAWING(S) - The figure is a perspective view of the core of the fuel processor in isolation from associated fluid supply and discharge pipework, as above.    Reaction zones (27/33)    Core (31)    Plates (32)    Catalyst receiving zones (35)