A process for producing a synthesis gas containing hydrogen and nitrogen, suitable for production of ammonia, wherein a raw synthesis gas (13) obtained by reforming of a natural gas feedstock is purified in a cryogenic separator (CS), and a portion of purified gas (16) is expanded and used as a cooling medium in the same separator, said expanded portion (16) being then re-introduced in the main stream of purified synthesis gas. A suitable apparatus and revamping of conventional plants according to the invention are also disclosed.