NOVELTY - A fuel processor for converting hydrocarbon fuel into hydrogen gas comprises at least two modules each configured to conduct distinct unit operation(s) required for reforming hydrocarbons in a fuel; a housing for housing the modules together; and an interstitial space within the housing juxtapositioned to the individual modules and an inner surface of the housing.    USE - Converting hydrocarbon fuel into hydrogen gas e.g. in a fuel cell.    ADVANTAGE - The processor provides more flexibility in selecting the physical shapes of units, e.g. monolithic catalyst supports. It provides better serviceability while retaining a very compact fuel processor. Reactors or modules can be changed out very quickly and replaced as opposed to having to dismantle an entire fuel processor assembly. The processor utilizes the interstitial space as a conduit for flowing a heat exchange medium, including a process gas, for thermal integration of the modules.    DETAILED DESCRIPTION - A fuel processor for converting hydrocarbon fuel into hydrogen gas comprises:    (a) at least two modules (12a, 12b) each configured to conduct distinct unit operation(s) required for reforming hydrocarbons in a fuel;    (b) a housing (14) for housing the modules together; and    (c) an interstitial space (24) within the housing juxtapositioned to the individual modules and an inner surface (26) of the housing.    The modules are non-concentrically aligned with respect to one another. The interstitial space is configured to provide functions(s) selected from:    (i) conducting a fluid through the space for heating the modules,    (ii) conducting a fluid through the space for cooling the modules,    (iii) conducting a fluid through the space for preheating a fluid,    (iv) conducting a fluid through the space and providing a catalyst in the space for reaction,    (v) providing an insulating non-gaseous material in the space for insulating the modules,    (vi) co-housing one or more monolithic catalyst supports and/or    (vii) co-housing one or more granular catalyst supports.    INDEPENDENT CLAIMS are also included for the following:    (1) a method of reforming hydrocarbon fuels, comprising flowing a feed stream in a first direction; generating a reformate from a first unit operation; flowing the reformate in a second direction opposite the first direction; conducting a second unit operation on the reformate; and simultaneously exchanging heat in the interstitial space about a system module via fluid flow through among: a heat exchange fluid flowing in either one of the first or second directions, and the first and second unit operations; and    (2) a method of constructing a fuel processor, comprising providing at least two modules configured to conduct at least one unit operation each; aligning the modules non-concentrically; housing the modules in the housing; and securing each module by its opposite ends to an end closure (30, 32) of the housing.    DESCRIPTION OF DRAWING(S) - The figure is a first perspective, partially exploded view of a fuel processor.    Modules (12a, 12b)    Housing (14)    Interstitial space (24)    Inner surface (26)    End closure (30, 32)