Emission of terahertz radiation from a AlGaN/GaN/Al $_{\mathbf {2}} \mathbf {O} _{\mathbf {3\, }}$ heterostructure with a surface metal grating is investigated under conditions of 2D electron heating in a lateral electric field. The studies are performed under essentially nonequilibrium conditions when the effective temperatures of 2D electrons and 2D plasmons are several times higher than the lattice temperature. This makes it possible to observe and explore high-quality peaks of intense THz radiation corresponding to 2D plasmon resonance.