An axisymmetric nonisobaric supersonic jet mixing with the atmosphere or expanding into a vacuum is considered as a model of a rocket engine plume. The expanding caused by the nonisobarity, turbulent mixing and afterburning are main processes forming a plume structure at different altitudes. Optical characteristics of the plume are determined by thermal radiation of condense-phase particles and vibration-rotation transitions in molecules. In the visible and ultraviolet spectral ranges chemiluminescent mechanisms make significant contribution to plume radiation. The Navier-Stokes equations with additional terms describing chemical kinetics and vibrational relaxation have been used for numerical simulation of plumes. An original technique has been developed to calculate selective radiation transfer in nonuniform nonequilibrium medium. Calculated plume spectra of typical liquid and solid fuel rocket engines at different altitudes are presented for infrared, visible and ultraviolet ranges.