NOVELTY - A microreactor is operated by providing fluid processing device; passing fluid through reaction microchamber (52); and transferring heat from exothermic chemical process to fluid flowing through a path.    USE - The invention is used in the operation of microreactor for performing equilibrium-limited reactions such as chemically reversible reactions or multiple competing reactions.    ADVANTAGE - The invention provides reactor with improved temperature control and enables better and more precise control of reaction temperatures. It provides improved conversion and/or selectivity in chemical reactions. It provides chemical reactor system that is compact and thermodynamically and/or energetically efficient. It provides reactor system requiring reduced catalyst loads and with reduced temperature gradients across the catalyst. It further provides reactor system with high heat transfer power densities.    DETAILED DESCRIPTION - The operation of microreactor involves providing fluid processing device having stack of thin sheets integrally bonded, the stack including alternating recessed sheets that define a portion of first and second flow paths, the first flow path having reaction microchamber with catalysts material (75) and having inlet (53) and outlet (55) within the stack, the second flow path is in thermal contact with the reaction microchamber; passing the fluid through the reaction microchamber in which the first fluid interacts with the catalyst material to undergo an exothermic chemical process in the microchamber; and transferring heat from the exothermic chemical process to the fluid flowing through the second path to cause temperature of the first fluid to be lower at the outlet than at the inlet to increase performance parameter(s) of the process.    INDEPENDENT CLAIMS are also included for:    (1) a method of performing an equilibrium limited exothermic chemical process comprising flowing reactants through a reaction microchamber in thermal contact with heat exchange channel(s); and conducting heat generated by the process into fluid flowing through heat exchange channel (61) to lower the temperature of the reactants as they progress through the microchamber by at least 25degreesC to increase the performance of the process;    (2) a method for performing a reversible exothermic reaction comprising flowing reactants through reaction microchamber in thermal contact with inlet and outlet portions of the heat exchange channel; and transferring heat generated by the reaction into the fluid flowing through the channel;    (3) a method for performing water gas shift reaction on product of fuel reforming process comprising providing reaction mixture containing carbon monoxide (CO), carbon dioxide (CO2), water (H2O), and hydrogen (H2) into reaction microchamber in thermal contact with the heat exchange channel(s) through an intermediate wall portion and a volume of the microchamber; and catalytically converting CO to CO2 in the microchamber at 50 mmol/hour of heat exchange core volume and transferring heat out of the microchamber such that the fluid exiting it contains less than 2 mol% CO and is colder than the fluid entering the microchamber;    (4) a method for performing endothermic and reversible exothermic reaction comprising flowing endothermic reactants through an endothermic reaction microchamber in thermal contact with the exothermic reaction microchamber to transfer heat from the exothermic reaction to sustain the endothermic reaction in which heat is transferred to raise the temperature of the endothermic reactants as they travel through the endothermic microchamber;    (5) a device comprising the stack of thin sheets integrally bonded; and    (6) a differential temperature microchannel chemical processing device comprising reaction microchamber having inlet and outlet ends and including exothermic reaction catalyst; heat exchange microchannel(s) in thermal contact with the reaction microchamber; and heater in thermal contact with the inlet end of the microchamber.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of a microchannel reactor.    Reaction microchamber (52)    Reaction flow path (51)    Inlet (53)    Outlet (55)    Heat exchange channel (61)    Walls (70, 72)    Catalyst (75)    Microchannel reactor (100)