<p>This paper addresses to the solving of the nonlinear radiative transfer equation in resonance lines under partial frequency redistribution. The existence of a constructive solution to the two-dimensional integral equation is proven for the general case of nonlinearity with non-coherent scattering. Special attention is focused to different kinds of nonlinear dependencies of the source function on the mean intensity. Along with nonlinearity, we also separately consider the cases of coherent, complete, and general frequency redistribution. Based on the theoretical results, numerical simulations are implemented to demonstrate the qualitative and quantitative differences between the linear and nonlinear solutions. Numerical calculations are carried out for the different spectral lines arising in masers, as well as in solar and stellar chromospheres.</p>