A method of treating an industrial waste gas generated in a chemical process and comprising at least one oxidizable substance, the method comprising: forwarding the industrial waste gas to an oxyfuel boiler plant, characterised in that the waste gas comprises at least 50% by volume of carbon dioxide, and by injecting the industrial waste gas into an oxyfuel boiler of the oxyfuel boiler plant and making the industrial waste gas participate in the combustion process occurring in the oxyfuel boiler to cause oxidation of at least a portion of the content of the at least one oxidizable substance of the industrial waste gas, generating a carbon dioxide rich flue gas in the oxyfuel boiler as an effect of the combustion process, and cleaning the carbon dioxide rich flue gas in a gas cleaning system of the oxyfuel boiler plant to form an at least partly cleaned carbon dioxide rich flue gas the method further comprising: utilizing at least a portion of the industrial waste gas as a first flow of dilution gas and mixing at least a portion of the industrial waste gas with a fuel prior to injecting the at least a portion of the industrial waste gas mixed with the fuel into the oxyfuel boiler; and. utilizing at least a portion of the industrial waste gas as a second flow of dilution gas and mixing the at least a portion of the industrial waste gas with an oxygen gas to form an oxidant gas mixture, and injecting the oxidant gas mixture into the oxyfuel boiler for combusting a fuel therein. forwarding the industrial waste gas to an oxyfuel boiler plant, characterised in that the waste gas comprises at least 50% by volume of carbon dioxide, and by injecting the industrial waste gas into an oxyfuel boiler of the oxyfuel boiler plant and making the industrial waste gas participate in the combustion process occurring in the oxyfuel boiler to cause oxidation of at least a portion of the content of the at least one oxidizable substance of the industrial waste gas, generating a carbon dioxide rich flue gas in the oxyfuel boiler as an effect of the combustion process, and cleaning the carbon dioxide rich flue gas in a gas cleaning system of the oxyfuel boiler plant to form an at least partly cleaned carbon dioxide rich flue gas the method further comprising: utilizing at least a portion of the industrial waste gas as a first flow of dilution gas and mixing at least a portion of the industrial waste gas with a fuel prior to injecting the at least a portion of the industrial waste gas mixed with the fuel into the oxyfuel boiler; and. utilizing at least a portion of the industrial waste gas as a second flow of dilution gas and mixing the at least a portion of the industrial waste gas with an oxygen gas to form an oxidant gas mixture, and injecting the oxidant gas mixture into the oxyfuel boiler for combusting a fuel therein. utilizing at least a portion of the industrial waste gas as a first flow of dilution gas and mixing at least a portion of the industrial waste gas with a fuel prior to injecting the at least a portion of the industrial waste gas mixed with the fuel into the oxyfuel boiler; and. utilizing at least a portion of the industrial waste gas as a second flow of dilution gas and mixing the at least a portion of the industrial waste gas with an oxygen gas to form an oxidant gas mixture, and injecting the oxidant gas mixture into the oxyfuel boiler for combusting a fuel therein.