A method for producing methanol, the method comprising: a first step of preparing mixed gas by using steam and natural gas as raw materials and converting C 2+ hydrocarbons contained in the natural gas into methane over a catalyst; a second step of preparing a synthesis gas including carbon monoxide, carbon dioxide, and hydrogen by reforming the mixed gas in a reformer filled with a reforming catalyst; and a third step of a methanol synthesis process, in which methanol is prepared by using the synthesis gas as a raw material and reacting the synthesis gas in a methanol synthesis reactor filled with a methanol synthesis catalyst, wherein the methanol product in the methanol synthesis process and unreacted synthesis gas are separated, some of the separated unreacted synthesis gas is supplied to a heating furnace, providing reaction heat required for the reformer of the second step and combusted together with the natural gas, and the mixed gas supplied to the reformer passes through a one first heat exchanger (E-105A, E-105B; E-105) while post-combustion gas emitted from the heating furnace passes through the one first heat exchanger (E-105A, E-105B; E-105) to supply the mixed gas heated through heat exchange with the post-combustion gas in the one first heat exchanger (E-105A, E-105B; E-105) to the reformer, wherein the carbon dioxide required for the methanol synthesis process is injected into any one of the reformer of the second step and the methanol synthesis reactor of the third step or is split to be injected into both of the reformer and the methanol synthesis reactor, and wherein while the post-combustion gas passes through a further heat exchanger (E-103), the natural gas and the steam as the raw materials supplied to a pre-reformer in which the mixed gas is prepared in the first step pass through the further heat exchanger (E-103) to supply the raw materials heated through the heat exchange with the post-combustion gas in the further heat exchanger (E-103) to the pre-reformer. a first step of preparing mixed gas by using steam and natural gas as raw materials and converting C 2+ hydrocarbons contained in the natural gas into methane over a catalyst; a second step of preparing a synthesis gas including carbon monoxide, carbon dioxide, and hydrogen by reforming the mixed gas in a reformer filled with a reforming catalyst; and a third step of a methanol synthesis process, in which methanol is prepared by using the synthesis gas as a raw material and reacting the synthesis gas in a methanol synthesis reactor filled with a methanol synthesis catalyst, wherein the methanol product in the methanol synthesis process and unreacted synthesis gas are separated, some of the separated unreacted synthesis gas is supplied to a heating furnace, providing reaction heat required for the reformer of the second step and combusted together with the natural gas, and the mixed gas supplied to the reformer passes through a one first heat exchanger (E-105A, E-105B; E-105) while post-combustion gas emitted from the heating furnace passes through the one first heat exchanger (E-105A, E-105B; E-105) to supply the mixed gas heated through heat exchange with the post-combustion gas in the one first heat exchanger (E-105A, E-105B; E-105) to the reformer, wherein the carbon dioxide required for the methanol synthesis process is injected into any one of the reformer of the second step and the methanol synthesis reactor of the third step or is split to be injected into both of the reformer and the methanol synthesis reactor, and wherein while the post-combustion gas passes through a further heat exchanger (E-103), the natural gas and the steam as the raw materials supplied to a pre-reformer in which the mixed gas is prepared in the first step pass through the further heat exchanger (E-103) to supply the raw materials heated through the heat exchange with the post-combustion gas in the further heat exchanger (E-103) to the pre-reformer.