Summary form only given, as follows. A recent theoretical study of the gyrotron backward-wave oscillator (gyro-BWO) showed that the axial field profile contracts in the nonlinear regime, which therefore suggests the possibility of stationary (saturated) operation at a beam current as high as two orders of magnitude above the self-modulation threshold predicted by early theories. A 3D simulation of this effect as well as the self-modulation effect resulting from the overdrive of the gyromonotron is being performed with the MAGIC3D code. Experimental verification of the self-modulation behavior of the gyromonotron and the much broader stationary operating range of gyro-BWO will be presented in a companion abstract. In this talk, comparisons of the simulation results and experimental data will be presented.