An ammonia synthesis gas mixture comprised of substantially pure hydrogen and nitrogen in approximately 3/1 molar ratio is obtained form two stage reforming of a methane-rich hydrocarbon charge such as natural gas. About 40% of the fresh natural gas is charged to primary steam reforming and the obtained primary reformate product, containing about 17 volume percent of unreacted methane, is mixed with the other 60% of the fresh natural gas and the mixture is subjected to oxidative reforming with enriched air containing 30 to 35 of O.sub.2, said enriched air being supplied in an amount to produce a secondary reformate containing principally hydrogen and nitrogen, accompanied by a minor amount of oxides of carbon (CO and CO.sub.2). The secondary reformate subjected to shift reaction with steam results in conversion of contained CO to CO.sub.2, thereby releasing hydrogen in an amount which together with the hydrogen previously present in said reformate obtains a H.sub.2 /N.sub.2 molar ratio of about 3/1. Carbon dioxide is removed and recovered from the shift reaction product by pressure swing adsorption in a first adsorbent bed. The other contaminants remaining are largely removed by further adsorption in second adsorbent bed in flow-communication with the first bed, the unadsorbed effluent being a syngas mixture consisting essentially of hydrogen and nitrogen in stoichiometric ratio for production of NH.sub.3.