A gas treatment method including an oxidative process providing waste heat, wherein a first gas mixture containing an oxidable component is provided, wherein the first gas mixture and oxygen are introduced into a oxidating unit, wherein at least a part of the oxidable component in the first gas mixture is oxidised in the oxidating unit, wherein the first gas mixture and/or a second gas mixture used in providing the first gas mixture and/or a third gas mixture produced from at least a part of the first gas mixture using the oxidating unit is or are treated in one or more gas treatment units operated using a first amount of steam, characterized by a first mode of operation and a second mode of operation, - the second mode of operation being a start-up mode and the first mode of operation being a regular mode of operation performed in a time period after the second mode of operation, - the oxidative process providing waste heat being in operation and providing waste heat in a first waste heat amount in the first mode of operation and the oxidative process being not in operation or being in operation and providing waste heat in a second waste heat amount smaller than the first waste heat amount in the second mode of operation, - in the first mode of operation the oxygen introduced into the oxidating unit being at least in part provided in the form of oxygen contained in an air separation product by an air separation unit, the air separation product being pure oxygen or a mixture of components enriched in oxygen as compared to atmospheric air, - a first amount of the air separation product being produced by the air separation unit in the first mode of operation, and a second amount of the air separation product smaller than the first amount of the air separation product or none of the air separation product being produced by the air separation unit in the second mode of operation, - the air separation unit comprising a compressor operated using a second amount of steam being expanded in a steam turbine mechanically coupled to the compressor, - in the first mode of operation the first amount of steam being provided by a first steam generator operated using at least a part of the waste heat, and in the first mode of operation the second amount of steam being provided by a second steam generator independently operable from the waste heat, and - in the second mode of operation at least a part of the first amount of steam provided by the first steam generator in the first mode of operation is instead provided by the second steam generator. - the second mode of operation being a start-up mode and the first mode of operation being a regular mode of operation performed in a time period after the second mode of operation, - the oxidative process providing waste heat being in operation and providing waste heat in a first waste heat amount in the first mode of operation and the oxidative process being not in operation or being in operation and providing waste heat in a second waste heat amount smaller than the first waste heat amount in the second mode of operation, - in the first mode of operation the oxygen introduced into the oxidating unit being at least in part provided in the form of oxygen contained in an air separation product by an air separation unit, the air separation product being pure oxygen or a mixture of components enriched in oxygen as compared to atmospheric air, - a first amount of the air separation product being produced by the air separation unit in the first mode of operation, and a second amount of the air separation product smaller than the first amount of the air separation product or none of the air separation product being produced by the air separation unit in the second mode of operation, - the air separation unit comprising a compressor operated using a second amount of steam being expanded in a steam turbine mechanically coupled to the compressor, - in the first mode of operation the first amount of steam being provided by a first steam generator operated using at least a part of the waste heat, and in the first mode of operation the second amount of steam being provided by a second steam generator independently operable from the waste heat, and - in the second mode of operation at least a part of the first amount of steam provided by the first steam generator in the first mode of operation is instead provided by the second steam generator.