The solution comprises a method of and a system for maintaining steam temperature and therefore electricity production efficiency with decreased loads of a steam turbine power plant comprising a fluidized bed boiler (12) and a fluidized bed superheater (2) adapted to superheat steam supplied to a steam turbine (3). According to the solution, the steam temperature may be maintained by providing, outside a furnace (10), additional heating to the fluidized bed material in its outer circulation, thereby increasing the amount of thermal energy available in the fluidized bed material to be transferred in the fluidized bed superheater (2) to the steam supplied to the steam turbine (3). Such additional heating may be performed by selectably supplying combustible gas with nozzles (111) into and/or burned with a burner in or in the vicinity of the fluidized bed material outside the furnace (10). As an additional aspect of the disclosed solution, the combustible gas may be produced with a gasifier (4).