  Based on the Boltzmann transport theory, we study the origin of the anomalous
temperature dependence of the Nernst coefficient ($\nu$) due to the phonon-drag
mechanism. For narrow-gap semiconductors, we find that there are two
characteristic temperatures at which a noticeable peak structure appears in
$\nu$. Contrarily, the Seebeck coefficient ($S$) always has only one peak.
While the breakdown of the Sondheimer cancellation due to the
momentum-dependence of the electron relaxation time is essential for the peak
in $\nu$ at low $T$, the contribution of the valence band to the phonon-drag
current is essential for the peak at higher $T$. By considering this mechanism,
we successfully reproduce $\nu$ and $S$ of FeSb$_2$ for which a gigantic
phonon-drag effect is observed experimentally.
