To be able to predict the IR-signature of an aircraft, the heat radiated, in form of IR-radiation, has to be calculated. A program package, SIGGE, has been developed for this purpose. The already existing code has been improved. The code can now handle radiation from hot gases and from walls with complex geometries, including specular reflections. This report describes some key equations used and a detailed description of SIGGE is presented. The code has been verified against other calculations with satisfactory result.