Crude ammonia synthesis gas is obtained by primary steam reforming of a hydrocarbon gas mixture rich in methane, such as natural gas, followed by secondary reforming of the primary reformate with added air and shift conversion of the contained CO to CO.sub.2. The shift conversion product is first freed of contained CO.sub.2 by selective absorption in a novel PSA unit having an integrated B section for removal of remaining impurities such as carbon monoxide and methane, thereby providing as product a gas stream comprised of hydrogen and nitrogen in approximate 3:1 molar ratio accompanied by a small amount or argon derived from the air stream used in the secondary reforming step. Alternative embodiments are disclosed for removal of CO from the gas stream, before its entry into the NH.sub.3 conversion operation; any residual CO that might have slipped thorugh the absorbent bed of the B section is converted to CH.sub.4.