Abstract: The invention relates to a method and a device for producing synthesis gas and for operating an internal combustion engine therewith. The engine fuel gases can be produced from changing heterogeneous organic fuels, wherein highly tar-laden product gas and pyrolysis gas are formed. By combining and purifying these gases, the gas mixtures or the syntheses gas formed and by special operation of the gas engine, tars and pyrolysis oil produced in a high proportion are usable as an addition to the ignition oil. The synthesis gas is generated from the pyrolysis gas that is generated in a pyrolysis reactor (2) and from the product gas that results from autothermal gasification of the pyrolysis coke formed and/or of the pyrolysis oil in an atmospheric steady-state fluidisedbed gasifier (8).