NOVELTY - Converting methane into a liquid mixture of (C3+) alkanes having 3 carbon atoms and more, preferably (C5+) alkanes having 5 carbon atoms and more, comprises: (1) converting methane into acetylene, carried out by e.g. submitting methane to a thermal cracking reaction to form a reaction mixture (M1); (2) converting acetylene into ethane, carried out by e.g. contacting acetylene or reaction mixture (M1) to form a reaction mixture (M2); (3) carrying out simultaneous ethane self- and cross-metathesis reactions to form a reaction mixture (M3); and (4) fractionating the reaction mixture (M3).    USE - The process is useful for producing hydrogen.    ADVANTAGE - The process is carried out by a non-oxidizing route, which is simple and direct, and relatively inexpensive, thus it is directly used on the natural gas deposit sites where methane is generally issued. The process is carried out with an extremely high carbon atom efficiency, which approaches 95% or higher for converting methane into liquid alkane mixtures, which is easily transportable in conditions relatively close to standard temperature and pressure conditions.    DETAILED DESCRIPTION - Converting methane into a liquid mixture of (C3+) alkanes having 3 carbon atoms and more, preferably (C5+) alkanes having 5 carbon atoms and more, comprises: (1) converting methane into acetylene, carried out by submitting methane to a thermal cracking reaction to form a reaction mixture (M1) comprising acetylene and hydrogen, and optionally by fractionating the reaction mixture (M1) to separate the acetylene; (2) converting acetylene into ethane, carried out by contacting the acetylene or the reaction mixture (M1) previously obtained with hydrogen in the presence of a hydrogenation catalyst (C1) to form a reaction mixture (M2) comprising ethane, and optionally by fractionating the reaction mixture (M2) to separate the ethane; (3) carrying out simultaneous ethane self- and cross-metathesis reactions by contacting the ethane or the reaction mixture (M2) previously obtained with a metal catalyst (C2) capable of producing, in contact with alkane, reactions involving the splitting and recombining of carbon-carbon, carbon-hydrogen and/or carbon-metal bonds to form a reaction mixture (M3) comprising methane and a mixture of the (C3+), preferably (C5+) alkanes; and (4) fractionating the reaction mixture (M3) to separate and to isolate the mixture of (C3+), preferably (C5+) alkanes, in liquid form.