The paper presents a software package for thermal computations of synchronous generators. The programs included in this package are applied when working out modernisation projects of synchronous generators for ,,ENERGOSERWIS" S.A. They enable carrying out thermal computations of the main construction elements of generators. They were used for working out quite a few modernisation projects of synchronous generators of 30 MW up to 500 MW aiming at increase in the rated power, replacement of the cooling medium (e.g. hydrogen by air), increase in the life and improvement of the weakest construction nodes. The use of the developed computer programs is illustrated by the example of a TGH-120 generator modernisation aiming at increase in its rated power up to 130 MW. Also the implementation of the TGH-120 generator modernisation to the manufacturing process in ,,ENERGOSERWIS" S.A. is presented in the paper. The measurement results are compared with the computation results.