NOVELTY - A hydrogen-containing product gas is generated in a reformer (10) by firing reforming radiant section to generate a flue gas that will heat radiant section catalyst-filled tubes and convection section (2), feeding mixed feed stream (1) comprising hydrocarbon feedstock and steam through convection section, feeding obtained preheated mixed feed stream to pre-reformer (7) or insulated vessel filled with pre-reforming catalyst, feeding a generated mixed feed into fired heater (8) then to radiant section catalyst-filled tubes. The reformer and the fired heater are not located within the reformer.    USE - For generating a hydrogen-containing product gas, preferably a synthesis gas, in a steam-methane reformer or a modular partition reformer.    ADVANTAGE - The process facilitates high incremental capacity expansion of existing syngas-based plants; and expands the production capacity of existing reformer through preheat and preforming without disrupting the output or energy balance of the reformer or any associated plant in a cost-effective manner. The fired heater may be relatively small and may have smaller heat duty compared to the reformer, thus the rate of the production of hydrogen-containing product gas can be varied in a matter of hours. When the fired heater is taken off-line, e.g. for servicing, the pre-reformer effluent may be fed directly to the reformer radiant section catalyst tubes, thus avoiding a complete shut-down of the reformer operation.    DETAILED DESCRIPTION - The pre-reformer receives the preheated mixed feed stream from the reformer convection section and feeds the pre-reformed and preheated mixed feed stream to the fired heater, which feeds the preheated and pre-reformed mixed feed stream at reheat temperature (600-700degreesC) to the inlet of the radiant section catalyst tubes (5).    An INDEPENDENT CLAIM is included for a process for increasing the hydrogen-containing product gas-generating capacity of a reformer comprising radiant section, reforming catalyst-filled tubes, transition section, and    DESCRIPTION OF DRAWING(S) - The drawing shows a process of generating a hydrogen-containing product gas in a reformer.    Mixed feed stream (1)    Convection section (2)    Radiant section catalyst tubes (5)    Pre-reformer (7)    Fired heater (8)    Reformer (10)