NOVELTY - An annular reactor assembly comprises fins extending at least partially in inner area and/or annular area and oriented in a longitudinal direction along a circumference of inner and/or outer tubes, and a control mechanism for selectively controlling heat transfer between fluids passing through the inner area and an annular area, respectively.    USE - For exchanging heat between at least two separate fluid streams useful in fuel processing systems.    ADVANTAGE - The device is lighter, more compact, and more efficient; and allows other annular in-line systems to be easily incorporated and controlled, and variable or selective control of the heat-exchange processes. It eliminates complicated plenums and piping required by other configurations, e.g. plate heat exchangers, and sealing problems.    DETAILED DESCRIPTION - An annular reactor assembly comprises:    (1) an inner cylinder forming an inner area;    (2) an outer cylinder surrounding the inner cylinder and forming an annular area;    (3) fins extending at least partially into the inner area and/or annular area and oriented in a longitudinal direction along a circumference of the inner and/or outer tubes; and    (4) a control mechanism (20) for selectively controlling heat transfer between two fluids passing through the inner area and an annular area, respectively. The control mechanism is connected to a terminal end of the inner and/or outer cylinders (120, 140).    INDEPENDENT CLAIMS are also included for:    (a) a method of constructing an annular reactor assembly comprising arranging inner and outer cylinders to create an annular area between them, providing manifold and flow control devices to permit two fluids to flow through inner space and annular space, and sealing the assembly to prevent an unwanted loss of the first fluid or second fluid and prevent mixing of the fluids; and    (b) an in-line annular reactor system comprising a control mechanism for selectively controlling heat transfer between fluids, an upstream annular module having an inner outlet and annular outlet, a downstream annular module having an inner inlet, and heat transfer module having an inner cylinder enclosing an inner area and an outer cylinder.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of the annular reactor assembly.    Control mechanism (20)    Inner and outer cylinders (120, 140)    Fins (180)    Central core structure (200)