NOVELTY - A fuel processor apparatus thermo-siphon boiler vessel provides an isothermal steam generator and heat removal system which improves the stability of the integrated reaction zone by providing a uniform temperature profile and strategic temperature control.    USE - For producing hydrogen used in conjunction with a fuel cell for the production of electricity.    ADVANTAGE - The apparatus provides a uniform temperature profile to avoid wide temperature swings and minimize mechanical and thermal stress. It provides an isothermal heat medium having a heat capacity greater than the feed or the reactor products to strategically control the temperature throughout the fuel processor zone.    DETAILED DESCRIPTION - A fuel processor apparatus comprises a first catalyst chamber (12) with a hollow interior containing upper (12c), center (12b), and lower zones (12a); a second catalyst chamber (14) annularly surrounding the first chamber; a burner chamber (22) annularly surrounding the second chamber; an insulation zone (28) arranged annularly about the burner zone; an air pre-heater zone arranged annularly the insulation zone; a third catalyst chamber (40) annularly arranged on the air pre-heater zone and arranged over the first chamber; steam conduits (54) arranged in a second catalyst zone (44) extending vertically through the second zone, top catalyst zone (42) and upper insulation zone (50); a boiler vessel (60) having a boiler interior (62); and a fluid gas exchange zone (64) arranged fully enclosing the boiler vessel and in intimate contact with the boiler vessel. The fluid gas exchange zone has a hot gas inlet (66) in fluid communication with upper flue gas outlet (26), a flue gas vent (68), and a cooled gas outlet (72). The boiler vessel has fluid inlet (90) and outlet (91). The third chamber has a top catalyst zone in fluid communication with upper zone of the first chamber, a second annular catalyst zone, a shift effluent conduit (46), and upper insulation zone. The air pre-heater zone comprises an air inlet conduit (32) and a pre-heater air outlet (34). The burner zone has a lower burner inlet (24) and an upper flue gas outlet. At least a portion of the second chamber is intimate thermal contact with the first chamber. The second chamber has an annular catalyst zone (16), a vertical feed conduit (18) at a top end and a fluid distributor/mixer (20) at a lower end.    An INDEPENDENT CLAIM is also included for a process for controlling a steam-to-hydrocarbon ratio of a feed stream to an apparatus for generating a hydrogen product gas stream for a fuel cell.    DESCRIPTION OF DRAWING(S) - The figure shows a vertical cross-sectional view of the apparatus.    First catalyst chamber (12)    Lower zones (12a)    Center zone (12b)    Upper zone (12c)    Second catalyst chamber (14)    Annular catalyst zone (16)    Vertical feed conduit (18)    Fluid distributor/mixer (20)    Burner chamber (22)    Lower burner inlet (24)    Upper flue gas outlet (26)    Insulation zone (28)    Air inlet conduit (32)    Pre-heater air outlet (34)    Third catalyst chamber (40)    Top catalyst zone (42)    Second catalyst zone (44)    Shift effluent conduit (46)    Upper insulation zone (50)    Steam conduits (54)    Boiler vessel (60)    Boiler interior (62)    Fluid gas exchange zone (64)    Hot gas inlet (66)    Flue gas vent (68)    Flue gas damper (70)    Cooled gas outlet (72)    Insulation (74)    Fourth catalyst zone (76)    Second effluent conduit (78)    Fluid level control device (82)    Fluid inlet (90)    Outlet (91)