Abstract: A retrofit method for increasing production capacity of an ammonia plant having a front end including in series primary and secondary reformers and a shift converter for reacting a hydrocarbon feed, steam and air to form a make-up syngas stream comprising hydrogen and nitrogen at about design stoichiometry, and a synthesis loop wherein a recycle syngas stream is combined with the make-up gas to form a syngas feed to ammonia converters. The retrofit involves the installation of an air separation unit to supply oxygen and nitrogen streams. The oxygen is used to enrich air supplied to the secondary reformer and increase the hydrogen content of the make-up gas substantially above the design stoichiometry and capacity. The nitrogen stream is supplied to the synthesis loop to obtain a desired hydrogen to nitrogen ratio in the syngas feed to the ammonia converters and compensate for the excess hydrogen in the make-up gas.