Method for adjusting a natural gas temperature for a fuel supply line of a gas turbine engine and gas turbine

The method for adjusting a natural gas temperature for a fuel supply line (6) of a gas turbine engine (1) comprises 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, adjusting the natural gas temperature on the basis of the index.