  We study gauge coupling unification in supersymmetric $E_6$ models where an
additional $\mathrm{U}(1)'$ gauge symmetry is broken near the TeV scale and a
number of exotic matter fields from the $\bm{27}$ representations have
$O(\mathrm{TeV})$ mass. Solving the 2-loop renormalization group equations of
gauge couplings and a kinetic mixing coupling between the $\mathrm{U}(1)'$ and
$\mathrm{U}(1)_Y$ gauge fields, we find that the gauge couplings fall into the
non-perturbative regime below the GUT scale. We examine threshold corrections
on the running of gauge couplings from both light and heavy ($\sim$ GUT scale)
particles and show constraints on the size of corrections to achieve the
perturbative unification of gauge couplings.
