  We study the discovery prospects of a heavy neutral scalar arising from a
$U(1)_{B-L}$ extension of the Standard Model (SM) during the Large Hadron
Collider's high luminosity runs (HL-LHC). This heavy neutral scalar mixes with
the SM Higgs boson through a Higgs portal and interacts with the SM particles
with an interaction strength proportional to the sine of the mixing angle. The
mixing between the two Higgs bosons is constrained by direct and indirect
measurements. We choose an experimentally viable mixing angle and explore in
detail the $ZZ$ and $WW$ decay modes of the heavy Higgs boson. For the $ZZ$
case, we focus on the cleanest $4\ell$ and $2\ell 2j$ final states and find
that a heavy Higgs boson of mass smaller than 500 GeV can be discovered at the
HL-LHC. For the $WW$ decay mode, we analyze the $\ell jj \slashed{E}_T$
signature. We implement novel background reduction techniques in order to
tackle the huge background by performing both cut-based and multivariate
analyses. However, large backgrounds render this channel challenging. We
briefly discuss the discovery prospects of the heavy $Z'$-boson arising in this
model.
