  We calculate the phonon-drag contribution to the transverse (Nernst)
thermoelectric power $S_{yx}$ in a bismuth single crystal subjected to a
quantizing magnetic field. The calculated heights of the Nernst peaks
originating from the hole Landau levels and their temperature dependence
reproduce the right order of magnitude for those of the pronounced
magneto-oscillations recently reported by Behnia et al [Phys.Rev.Lett. 98,
166602 (2007)]. A striking experimental finding that $S_{yx}$ is much larger
than the longitudinal (Seebeck) thermoelectric power $S_{xx}$ can be naturally
explained as the effect of the phonon drag, combined with the well-known
relation between the longitudinal and the Hall resistivity
$rho_{xx}>>|rho_{yx}|$ in a semi-metal bismuth. The calculation that includes
the contribution of both holes and electrons suggests that some of the hitherto
unexplained minor peaks located roughly at the fractional filling of the hole
Landau levels are attributable to the electron Landau levels.
