  The next generation of giant-segmented mirror telescopes ($>$ 20 m) will
enable us to observe galactic nuclei at much higher angular resolution and
sensitivity than ever before. These capabilities will introduce a revolutionary
shift in our understanding of the origin and evolution of supermassive black
holes by enabling more precise black hole mass measurements in a mass range
that is unreachable today. We present simulations and predictions of the
observations of nuclei that will be made with the Thirty Meter Telescope (TMT)
and the adaptive optics assisted integral-field spectrograph IRIS, which is
capable of diffraction-limited spectroscopy from $Z$ band (0.9 $\mu$m) to $K$
band (2.2 $\mu$m). These simulations, for the first time, use realistic values
for the sky, telescope, adaptive optics system, and instrument, to determine
the expected signal-to-noise ratio of a range of possible targets spanning
intermediate mass black holes of $\sim10^4$ \msun to the most massive black
holes known today of $>10^{10}$ $M_\odot$. We find that IRIS will be able to
observe Milky Way-mass black holes out the distance of the Virgo cluster, and
will allow us to observe many more brightest cluster galaxies where the most
massive black holes are thought to reside. We also evaluate how well the
kinematic moments of the velocity distributions can be constrained at the
different spectral resolutions and plate scales designed for IRIS. We find that
a spectral resolution of $\sim8000$ will be necessary to measure the masses of
intermediate mass black holes. By simulating the observations of galaxies found
in SDSS DR7, we find that over $10^5$ massive black holes will be observable at
distances between $0.005 < z < 0.18$ with the estimated sensitivity and angular
resolution provided by access to $Z$-band (0.9 $\mu$m) spectroscopy from IRIS
and the TMT adaptive optics system. (Abridged)
