NOVELTY - The method comprises: measuring a natural gas percentage content of methane, ethane, propane, butane and carbon dioxide by an infrared analysis; calculating the nitrogen percentage content based on the measured percentage content of methane, ethane, propane, butane and carbon dioxide; calculating an index indicative of the natural gas energy content; and adjusting the natural gas temperature on the basis of the index. The step of adjusting the natural gas temperature includes defining a set of ranges for the index and associating one maximum temperature to each range.    USE - The method is useful for adjusting a natural gas temperature of a fuel supply line of a gas turbine engine (claimed) that is used in a combined cycle power plant.    ADVANTAGE - The method is capable of adjusting the natural gas temperature of the fuel supply line of the gas turbine engine in an efficient and reliable manner so as to define a maximum allowable fuel gas temperature, thus enabling continuous and reliable operation of the combined cycle power plant with fluctuating gas compositions at optimum combined cycle efficiency, and enabling a fast response of the gas turbine engine operation to changes in composition of the natural.    DETAILED DESCRIPTION - The method comprises: measuring a natural gas percentage content of methane, ethane, propane, butane and carbon dioxide by an infrared analysis; calculating the nitrogen percentage content based on the measured percentage content of methane, ethane, propane, butane and carbon dioxide; calculating an index indicative of the natural gas energy content; and adjusting the natural gas temperature on the basis of the index. The step of adjusting the natural gas temperature includes: defining a set of ranges for the index; associating one maximum temperature to each range; and maintaining the natural gas temperature below or at the maximum temperature corresponding to the range into which the calculated index falls. The index is calculated by: calculating the lower heat value of a mixture including the measured amount of methane, ethane, propane, butane and carbon dioxide and the calculated amount of nitrogen; calculating the molecular weight of the mixture including the measured amount of methane, ethane, propane, butane and carbon dioxide and the calculated amount of nitrogen; and calculating the index by an equation ((NGI = (LHVng/LHVch4)x (Mng/Mch4) one half ) or (NGI = (LHVng/LHVdes)x (Mng/Mdes) one half )), where the LHVng is the calculated lower heat value of the mixture, the LHVch4 is the lower heat value of methane, LHVdes is the design lower heat value, Mng is the molecular weight of the mixture, Mch4 is the molecular weight of methane and Mdes is the design molecular weight. A gas turbine engine (1) has a set of fuel supply lines (6a, 6b) for different combustion chambers (3, 15) and/or different stages of a combustion chamber, where the natural gas temperature of one of the fuel supply line is adjusted independently of the natural gas temperature of another fuel supply line. An INDEPENDENT CLAIM is included for a gas turbine engine.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a gas turbine engine.    Gas turbine engine (1)    Compressor (2)    Combustion chambers (3, 15)    Fuel supply lines (6a, 6b)    Heat exchanger. (7)