NOVELTY - A mixture of hydrogen and carbon monoxide is prepared from a gaseous mixture of hydrocarbons by heating gaseous mixture by indirect heat exchange between reactor effluent and pre-reformed gaseous mixture using printed circuit heat exchanger by feeding reactor effluent through first set of numerous microchannels present in printed circuit heat exchanger and feeding pre-reformed gaseous mixture through second set of numerous microchannels present in printed circuit heat exchanger.    USE - Process of preparing a mixture of hydrogen and carbon monoxide from a gaseous mixture of hydrocarbons for preparing Fischer-Tropsch synthesis product (claimed).    ADVANTAGE - The preparation of the mixture is simple, and employs a simple and low cost apparatus having smaller size.    DETAILED DESCRIPTION - A mixture of hydrogen and carbon monoxide is prepared from a gaseous mixture of hydrocarbons by:    (A) pre-reforming the hydrocarbon mixture comprising methane, ethane and optional higher carbon number hydrocarbons in the presence of steam and a reforming catalyst to convert ethane and optional higher carbon number hydrocarbons to methane, carbon dioxide and hydrogen;    (B) heating the gaseous mixture to a temperature greater than 600 degrees C;    (C) performing a partial oxidation by contacting the heated mixture with a source of oxygen in a reactor burner yielding a reactor effluent having a temperature of 1100-1500 degrees C; and    (D) reducing the temperature of the reactor effluent by indirect heat exchange against evaporating water to a temperature of 750-900 degrees C where the heating in step (b) is effected by indirect heat exchange between the effluent of step (d) and the pre-reformed gaseous mixture obtained in step (a) using a printed circuit heat exchanger by feeding the effluent of step (d) through a first set of numerous microchannels present in the printed circuit heat exchanger and feeding the pre-reformed gaseous mixture through a second set of numerous microchannels present in the printed circuit heat exchanger where the first and second set of microchannels are oriented so that convective heat exchange takes place between the first and second set of microchannels.    An INDEPENDENT CLAIM is included for preparation of Fischer-Tropsch synthesis product comprising subjecting hydrogen and carbon monoxide mixture to Fischer-Tropsch synthesis to convert the mixture into liquid hydrocarbons in the presence of a Fischer-Tropsch type catalyst comprising metal(s) selected from Group 8 of Periodic Table; and separating obtained product into a light stream and a heavy stream.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic view of a combined heat exchanger.    Desulfurized natural gas (15)    Gaseous by-products stream (16)    Pre-reformer reactor (19)    Pre-reformed effluent (20)    Partial oxidation reactor (22)