  NEXT-100 is an electroluminescent high-pressure xenon gas time projection
chamber that will search for the neutrinoless double beta ($\beta \beta 0 \nu$)
decay of Xe-136. The detector possesses two features of great value for $\beta
\beta 0 \nu$ searches: energy resolution better than 1\% FWHM at the $Q$ value
of Xe-136 and track reconstruction for the discrimination of signal and
background events. This combination results in excellent sensitivity, as
discussed in this paper. Material-screening measurements and a detailed Monte
Carlo detector simulation predict a background rate for NEXT-100 of at most
$4\times10^{-4}$ counts keV$^{-1}$ kg$^{-1}$ yr$^{-1}$. Accordingly, the
detector will reach a sensitivity to the \bbonu-decay half-life of
$2.8\times10^{25}$ years (90\% CL) for an exposure of 100
$\mathrm{kg}\cdot\mathrm{year}$, or $6.0\times10^{25}$ years after a run of 3
effective years.
