Apparatus for rapidly heating one or more reactants for use in a fuel reformer. The apparatus includes a combustion section having an outer wall enclosing a combustion chamber and a catalyst disposed within the combustion chamber. The catalyst provides a non-diffused flow path through the combustion chamber for the combustion of gases and generation of heat and passage of such gases and heat from the combustion chamber. A heat recovery section is in fluid communication with the combustion section and has an outer wall defining a heat recovery chamber. At least one heat exchanging element is located within the heat recovery chamber having an inlet for receiving a fuel reforming reactant and an outlet for directing a heated reactant out of the heat recovery section. The combustion section is elevated relative to the heat recovery section such that heated combustion gases are displaced down into the heat recovery section. The combustion and heat recovery sections have outer walls that are continuous with one another to form a housing that is preferably cylindrical. The housing has an internal diameter of less than about 10 inches, or alternatively, has a length and a diameter in a ratio of between about 7:1 and about 4:1. Methods for the rapid start-up of a fuel processor using such a pre-reformer are also provided, as are integrated fuel processors comprising a fuel reformer and the disclosed pre-reformer.