The method for adjusting a natural gas temperature for a fuel supply line of a gas turbine engine includes measuring by infrared analysis the natural gas percentage content of methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), carbon dioxide (CO2), calculating the nitrogen (N2) percentage content as the complement to 100 of the measured percentage content of methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), carbon dioxide (CO2), calculating an index indicative of the natural gas energy content and adjusting the natural gas temperature on the basis of the index.