A process and plant for producing a makeup synthesis gas (20) for the synthesis of ammonia, comprising: reforming of a hydrocarbon feedstock (10) in a steam reformer (SMR), purification of said gas product obtaining a hydrogen synthesis gas, addition of a suitable amount of nitrogen (19) to said hydrogen synthesis gas, wherein said purification includes separation of hydrogen in a main separation unit (S1), which produces a hydrogen current (14) and a tail gas (15), and wherein further hydrogen is recovered from the said tail gas (15) in an additional separation unit (S2); some embodiments provides that an amount of gas product (24) is also fed to said additional separation unit; a corresponding method for the revamping of a front-end of an ammonia plant is also disclosed.