NOVELTY - Providing an improved fuel conversion reactor including a shell-and-tube heat exchanger for controlling the temperature of the hot gaseous mixture following the reaction and controlling the temperature of the incoming gases prior to the reaction.    USE - For heating a gaseous fluid prior to reaction with a fuel and for cooling a gaseous mixture produced by the reaction; for converting a fuel to a hot gaseous mixture and as a fuel conversion reactor for converting hydrogen-containing fuel (e.g. petroleum fractions, natural gas or its component, and alcohol (preferably gasoline, kerosene, diesel fuel, naphtha, methane or propane)) to hydrogen using a catalytic reaction and also as fuel reformer (all claimed) and burners. For power generation and for the purpose of transportation.    ADVANTAGE - The novel reformer controls the temperature of a hot gaseous mixture produced by catalytic or non-catalytic reaction of a fuel with a gaseous fluid, and controls the temperature of the gaseous fluid and/or the fuel prior to the reaction.    DETAILED DESCRIPTION - A fuel conversion reactor comprises a shell-and-tube heat exchanger. The heat exchanger comprises: a primary shell member having primary and secondary ends and a sidewall extending between the ends and defining a heat exchanging chamber within the shell member; an inlet for introducing the gaseous fluid into the heat exchanging chamber; a first tube sheet fixed onto the primary shell member in the vicinity of the primary end and sealingly closing the heat exchanging chamber at one end of the chamber; a second tube sheet device which is separated from the primary shell member and is in the vicinity of the secondary end; and several heat exchange tubes extending from the first tube sheet to the second tube sheet device and is rigidly connected to both the first tube sheet and the second tube sheet device. The second tube sheet device forms another end of the chamber that is opposite the one end of the chamber. The heat exchange tube provides passageways for the gaseous mixture to flow inside the tubes through the heat exchanging chamber. At least one outlet aperture is formed in the region of the secondary end of the primary shell member to provide at least one outlet for the gaseous fluid, which flows through the heat exchanging chamber on a shell-side during operation of the fuel conversion reactor.    INDEPENDENT CLAIMS are also included for:    (1) a method of converting a fuel to a hot gaseous mixture by:    (a) providing a heat exchanging shell apparatus defining a heat exchanging chamber and having several heat exchange tubes mounted so that each extends through the chamber; the tubes provide passageways for flow of the hot gaseous mixture;    (b) introducing the gaseous fluid into the heat exchanging chamber, passing the gaseous fluid through the chamber, and causing the gaseous fluid to be heated by heat exchange with the hot gaseous mixture flowing through the tubes;    (c) withdrawing the heated gaseous fluid from the chamber and then mixing the heated gaseous fluid with the fuel to provide an initial mixture of the fuel and the gaseous fluid;    (d) producing the hot gaseous mixture from the initial mixture; and    (e) passing the hot gaseous mixture through the passageways, thus partially cooling the hot gaseous mixture to a lower temperature;    (2) a heat exchange apparatus for a fuel conversion reactor;    (3) a fuel reformer for converting hydrogen-containing fuel to hydrogen using a catalytic reaction; and    (4) a method of converting hydrogen-containing fuel to hydrogen using a catalytic reaction.