A method of revamping an existing ammonia plant fed with natural gas for increasing the capacity of said ammonia plant, wherein: said plant comprises a front-end section for converting natural gas into a make-up gas for ammonia synthesis, and said front-end section comprises a primary reformer and a secondary reformer, the overall supply of natural gas fed to the plant comprises a process quota which is used for the catalytic conversion of methane into hydrogen and is fed to said primary reformer, and a fuel quota, said primary reformer comprises a radiant section and a convective section, said radiant section performs a catalytic conversion of said process gas and transfers to said process gas a thermal power also called duty of the reformer, said secondary reformer is fed with comburent process air and with partially reformed process gas coming from the primary reformer, the method being characterised in that it comprises at least the following interventions: - decreasing said duty of the primary reformer relative to the amount of ammonia produced, and consequently decreasing the exit temperature of the partially reformed gas effluent from said primary reformer; - adding a feeding line directed to said secondary reformer for feeding pure oxygen and at least partially replacing said comburent process air; - adding addition a nitrogen injection line for injecting nitrogen in a quantity necessary to obtain said make-up gas suitable for ammonia synthesis; - reducing said fuel quota and consequently increasing said process quota, for a given supply of natural gas. said plant comprises a front-end section for converting natural gas into a make-up gas for ammonia synthesis, and said front-end section comprises a primary reformer and a secondary reformer, the overall supply of natural gas fed to the plant comprises a process quota which is used for the catalytic conversion of methane into hydrogen and is fed to said primary reformer, and a fuel quota, said primary reformer comprises a radiant section and a convective section, said radiant section performs a catalytic conversion of said process gas and transfers to said process gas a thermal power also called duty of the reformer, said secondary reformer is fed with comburent process air and with partially reformed process gas coming from the primary reformer, the method being characterised in that it comprises at least the following interventions: - decreasing said duty of the primary reformer relative to the amount of ammonia produced, and consequently decreasing the exit temperature of the partially reformed gas effluent from said primary reformer; - adding a feeding line directed to said secondary reformer for feeding pure oxygen and at least partially replacing said comburent process air; - adding addition a nitrogen injection line for injecting nitrogen in a quantity necessary to obtain said make-up gas suitable for ammonia synthesis; - reducing said fuel quota and consequently increasing said process quota, for a given supply of natural gas. - decreasing said duty of the primary reformer relative to the amount of ammonia produced, and consequently decreasing the exit temperature of the partially reformed gas effluent from said primary reformer; - adding a feeding line directed to said secondary reformer for feeding pure oxygen and at least partially replacing said comburent process air; - adding addition a nitrogen injection line for injecting nitrogen in a quantity necessary to obtain said make-up gas suitable for ammonia synthesis; - reducing said fuel quota and consequently increasing said process quota, for a given supply of natural gas.