  We present a method for the calculation of asymptotic operators for
nonequilibrium steady-state quantum systems. The asymptotic steady-state
operator is obtained by averaging the corresponding operator in Heisenberg
representation over infinitely long time. Several examples are considered to
demonstrate the utility of our method. The results obtained within our approach
are compared to those obtained within the Schwinger-Keldysh nonequilibrium
Green's functions.
