A fuel conversion reactor comprising a shell-and-tube heat exchanger for heating a gaseous fluid prior to reaction with a fuel and for cooling a gaseous mixture produced by the reaction, said heat exchanger comprising: a primary shell member having primary and secondary ends (32, 34) and a sidewall extending between said ends (32, 34) and defining a heat exchanging chamber located within the shell member; an inlet for introducing said gaseous fluid into said heat exchanging chamber; a first tube sheet fixedly mounted on said primary shell member in the region of said primary end and sealingly closing said heat exchanging chamber at one end of the chamber; a second tube sheet device (40, 64) which is separate from said primary shell member and is located in the region of said secondary end, said second tube sheet device (40, 64) forming another end of said chamber that is opposite said one end of the chamber; and a plurality of heat exchange tubes extending from said first tube sheet to said second tube sheet device (40, 64) and rigidly connected to both the first tube sheet and the second tube sheet device (40, 64), said heat exchange tubes providing passageways for said gaseous mixture to flow inside the tubes through said heat exchanging chamber; wherein one or more outlet apertures are formed at or in the region of said secondary end of said primary shell member in order to provide at least one outlet for said gaseous fluid which flows through said heat exchanging chamber on a shell-side thereof during operation of said fuel conversion reactor; characterised in that said fuel conversion reactor further comprises an outer shell (12, 112) having first and second ends (14, 16) and an outer shell wall extending between said first and second ends (14, 16), said outer shell being closed at said second end, extending around said primary shell member and said second tube sheet device (40, 64), and having an inlet for said fuel, wherein a fuel passageway is formed between said outer shell wall and said sidewall of the primary shell member and extends from said inlet for the fuel to said one or more outlet apertures; and in that said second tube sheet device (40, 64) includes a secondary shell member having a peripheral sidewall, a first end located adjacent said secondary end of the primary shell member and a second end spaced from the first end; in that the secondary shell member and the secondary end of the primary shell member together form a disconnected joint; and in that said second tube sheet device (40, 64) is free to move relative to said primary shell member upon thermal expansion of said heat exchange tubes. a primary shell member having primary and secondary ends (32, 34) and a sidewall extending between said ends (32, 34) and defining a heat exchanging chamber located within the shell member; an inlet for introducing said gaseous fluid into said heat exchanging chamber; a first tube sheet fixedly mounted on said primary shell member in the region of said primary end and sealingly closing said heat exchanging chamber at one end of the chamber; a second tube sheet device (40, 64) which is separate from said primary shell member and is located in the region of said secondary end, said second tube sheet device (40, 64) forming another end of said chamber that is opposite said one end of the chamber; and a plurality of heat exchange tubes extending from said first tube sheet to said second tube sheet device (40, 64) and rigidly connected to both the first tube sheet and the second tube sheet device (40, 64), said heat exchange tubes providing passageways for said gaseous mixture to flow inside the tubes through said heat exchanging chamber;