TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The combined system further comprises a coke oven plant, and a plant for generating hydrogen, connected to gas line system by a hydrogen-carrying line. The gas line system comprises a gas distribution for coke oven gas obtained in a coking process in the coke oven. The gas line system comprises a mixing device for producing a mixed gas containing blast furnace gas, converter gas and/or coke oven gas, arranged upstream the gas diverter. The plant for generating hydrogen is an electrolysis system for water electrolysis. The chemical plant comprises several parallel-connected smaller units, which are switched on or -off according to available stationary useful gas quantity power. Preferred Method: The method comprises mixing portion of blast furnace gas obtained during production of pig iron in the blast furnace and/or portion of converter gas obtained in the converter steel mill with a partial quantity of the coke oven gas produced in the coke oven, and utilizing the mixed gas as useful gas. The method comprises mixing blast furnace gas and converter gas, producing synthetic gas from the obtained mixed gas after gas conditioning, and mixing the synthetic gas or the purified mixed gas with purified coke oven gas before further processing to the synthetic gas. The method comprises producing synthetic gas from the blast furnace gas after gas conditioning, and mixing the synthesis gas or the purified blast furnace gas with purified coke oven gas before further processing to the synthetic gas. The method comprises producing synthetic gas from the converter gas, and mixing the synthesis gas or the purified converter gas with purified coke oven gas before further processing to the synthetic gas. The external current is obtained from renewable energy. The process variable is determined depending on functions including the price of external purchased current and the cost of production of the power station current as variables. The power of the power station is controlled depending on the useful gas-quantity supplied to the power station process of 20-100%. The power of the chemical plant or biotechnological plant is controlled depending on the mixed gas quantity supplied to the chemical plant or biotechnological plant.