NOVELTY - The method involves producing hot process gas by an inlet in a pyrolysis unit (50). A synthesis gas is produced by an outlet in the pyrolysis unit from a pyrolyzed feedstock. Process gas is derived from a part of the synthesis gas by a synthesis gas burner (90). The syngas burner is controlled once pyrolysis of the feedstock is commenced to maintain an inlet and outlet temperatures (T1, T2) within pre-determined limits by controlling a flow of syngas through the syngas burner, where the flow of the syngas is varied by adjusting speed of a variable speed fan (70) in the syngas burner.    USE - Method for operating a pyrolysis or gasification system of type for pyrolysising organic material to produce gas and liquid products/chemicals e.g. charcoal, activated carbon and methanol, in a gas turbine engine in a chemical industry from wood, to convert biomass into syngas and biochar and to turn waste into safely disposable substances, and for transforming medium-weight hydrocarbons from oil into petrol.    ADVANTAGE - The method enables operating the pyrolysis or gasification system of type for pyrolysising the organic material to produce the chemicals in the chemical industry from the wood in an easy and effective manner. The method enables ensuring that adequate process energy is delivered for thermal decomposition of the feedstock in a pyrolysis chamber to occur, while maintaining process gas temperature and the temperature of the syngas leaving the pyrolysis chamber within desired limits by controlling the burner to derive process gas from the syngas fed back from the pyrolysis chamber outlet, thus allowing a waste pyrolysis system to be operated in a controlled, near steady state despite variations in the feedstock, and hence improving throughput, reliability and efficiency of the system.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of a pyrolysis system.    Inlet and outlet temperatures (T1, T2)    Pyrolysis unit (50)    Cyclone (60)    Variable speed fan (70)    Synthesis gas burner (90)    Combustor (110)