Process for denitrogenation of natural gas with or without recovery of helium

In a natural gas denitrogenation by distillation process, natural gas (11, 12) cooled in an exchange line (1) is separated in a column system comprising at least one column (2, 3), a gas enriched nitrogen (18) is withdrawn from a column (3) of the column system and is heated in the heat exchange line, a methane-enriched liquid (13) is withdrawn from a column (3) of the column system, pressurized and vaporized in the exchange line to at least a vaporization pressure and at least a portion of the cooled natural gas expands in gaseous form in a turbine (7) and is fed to a column (2) of the column system under gaseous form.