NOVELTY - The method comprises: bringing a first inorganic starting material of the produced chemical product and a carbon-free second starting material such as hydrogen, of the chemical product together; providing an access to a source (30) of a carbon-containing compound (5) such as carbon dioxide; bringing a portion of the waste by-product hydrogen and the carbon-containing compound supplied from the source together; and carrying out catalytic methanation for the obtained mixture to convert the mixture into methane gas.    USE - The method is useful for producing a chemical product such as sodium hydroxide solution, in the form of two different components (claimed).    ADVANTAGE - The method is capable of producing the chemical product in an economical manner with high recycling value.    DETAILED DESCRIPTION - The method comprises: bringing a first inorganic starting material (1) of produced chemical product (3) and a carbon-free second starting material such as hydrogen, of the chemical product together, where a part of hydrogen in the second raw material (2) is reduced in molecular form as a byproduct of the manufacturing process; providing an access to a source (30) of a carbon-containing compound (5) such as carbon dioxide; bringing a portion of the waste by-product hydrogen and the carbon-containing compound supplied from the source together; and carrying out catalytic methanation for the obtained mixture to convert the mixture into methane gas. The source of carbon-containing compound is an air filtering device, a carbon dioxide and/or carbon monoxide-filter of an industrial plant and/or biogas plant. The source is partially added with exhaust gases of a thermal power plant, and comprises an intermediate storage for the exhaust gas. The methanation catalyst is carried out in two stages. The exhaust gases are fed into an existing gas network and/or a gas network. The hydrogen is brought to the pressure and/or temperature level at which methanation takes place. The hydrogen is produced in an amount of 3000 Nm3/h. The electrical power required for production process of the hydrogen is obtained from renewable power supplies. The method is carried out by chlorine-alkali electrolysis. An INDEPENDENT CLAIM is included for a plant for producing a chemical product in the form of two different components.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a plant for producing a chemical product.    Inorganic starting material (1)    Second raw material (2)    Chemical product (3)    Carbon-containing compound (5)    Source of carbon-containing compound. (30)