The cost functions of synthetic natural gas (SNG) produced by a conventional Lurgi process and by a modified Lurgi process are compared. The proposed modification consists in the suppression of the shift conversion and in the use of hydrogen to adjust the H//2/CO ratio before methanation. The condition that must be observed in order for the proposed scheme to be economically advantageous is established, and a parametric analysis is made of the SNG production costs of the two processes with reference to the main factors. 6 refs.