  The most direct way to confront observed galaxies with those formed in
numerical simulations is to forward-model simulated galaxies into synthetic
observations. Provided that synthetic galaxy observations include similar
constraints and limitations as real observations, they can be used to (1) carry
out even-handed comparisons of observation and theory and (2) map the
observable characteristics of simulated galaxies to their a priori known
origins. In particular, integral field spectroscopy (IFS) expands the scope of
such comparisons and mappings to an exceptionally broad set of physical
properties. We therefore present RealSim-IFS: a tool for forward-modelling
galaxies from hydrodynamical simulations into synthetic IFS observations. The
core components of RealSim-IFS model the detailed spatial sampling mechanics of
any fibre-bundle, image slicer, or lenslet array IFU and corresponding
observing strategy, real or imagined, and support the corresponding propagation
of noise adopted by the user. The code is highly generalized and can produce
cubes in any light- or mass-weighted quantity (e.g. specific intensity,
gas/stellar line-of-sight velocity, stellar age/metallicity, etc.). We show
that RealSim-IFS exactly reproduces the spatial reconstruction of specific
intensity and variance cubes produced by the MaNGA survey Data Reduction
Pipeline using the calibrated fibre spectra as input. We then apply RealSim-IFS
by producing a public synthetic MaNGA stellar kinematic survey of 893 galaxies
with $\log M_{\star}/M_{\odot}>10$ from the TNG50 cosmological hydrodynamical
simulation.
