NOVELTY - Operating a biogas plant, comprises supplying a defined amount of a liquid or moist fermentation substrate from a fermenter of the biogas plant (1) to a drying device (5), supplying the dried fermentation substrate to a synthesis gas device (6), in which the dried fermentation substrate is gasified to produce a synthesis gas and supplying the synthesis gases produced during the gasification to a gas engine (8). The liquid or moist fermentation substrate is fed from the fermenter (2) of the biogas plant to the drying device, without previous separation, which is carried out by a separator.    USE - The method is useful for operating a biogas plant.    ADVANTAGE - The method: exhibits optimized and efficient combustion of a synthesis gas and a functionally reliable operation of a gas engine; includes the liquid or moist fermentation substrate fed from at least one of the fermenter of the biogas plant to the drying device without a previous separation, by means of a separator, thus reduces the component costs and simplifies the process control; includes safe removal of the tar components from the synthesis gas, thus the synthesis gas is pure and lifetime of the synthesis gas engine is increased; and includes synthesis gas engine operated at optimum combustion temperature, thus requires less repair of the synthesis gas engine and increases its life time.    DETAILED DESCRIPTION - Operating a biogas plant, comprises supplying a defined amount of a liquid or moist fermentation substrate from at least one of the fermenter of the biogas plant to a drying device, supplying the dried fermentation substrate to a synthesis gas device, in which the dried fermentation substrate is gasified to produce a synthesis gas at a defined temperature and supplying a defined amount of the synthesis gases produced during the gasification to a gas engine. The liquid or moist fermentation substrate is fed from at least one of the fermenter of the biogas plant to the drying device, without previous separation, which is carried out by a separator. The liquid or moist fermentation substrate is dried in the drying device upto a defined moist residue exhibiting a dried substrate content of 70-90%, preferably 80-85%. A defined amount of the synthesis gas produced from the synthesis gas device is supplied to a synthesis gas engine, without mixing with the biogas produced in at least a fermenter (2) of the biogas plant. A synthesis gas engine is operated independently by burning biogas in biogas engine. An INDEPENDENT CLAIM is also included for the biogas plant for carrying out the above mentioned method comprising (i) at least a fermenter filled with the liquid or moist fermentation substrate and a drying device, in which a defined amount of the liquid or moist fermentation substrate is dried, (ii) a synthesis gas device, in which a defined amount of dried fermentation substrate is gasified, (iii) a gas engine, which is fed with defined amount of synthesis gas, where the drying device is downstream to at least a fermenter.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of a concrete structure of the biogas plant for carrying out the above mentioned process.    Biogas plant (1)    Fermenter (2)    Drying device (5)    Synthesis gas device (6)    Gas engine (8)