  We study hydrodynamic thermal transport in high-mobility two-dimensional
electron systems placed in an in-plane magnetic field, and identify a new
mechanism of thermal magnetotransport. This mechanism is caused by drag between
the electron populations with opposite spin polarization, which arises in the
presence of a hydrodynamic flow of heat. In high mobility systems, spin drag
results in strong thermal magnetoresistance, which becomes of the order of 100%
at relatively small spin polarization of the electron liquid. We express the
thermal magnetoresistance in terms of intrinsic dissipative coefficients of
electron fluid and show that it is primarily determined by the spin diffusion
constant.
