  The $^{18}$Ne($\alpha$,$p$)$^{21}$Na reaction is thought to be one of the key
breakout reactions from the hot CNO cycles to the rp-process in type I x-ray
bursts. In this work, the resonant properties of the compound nucleus $^{22}$Mg
have been investigated by measuring the resonant elastic scattering of
$^{21}$Na+$p$. An 89 MeV $^{21}$Na radioactive beam delivered from the CNS
Radioactive Ion Beam Separator bombarded an 8.8 mg/cm$^2$ thick polyethylene
(CH$_{2}$)$_{n}$ target. The $^{21}$Na beam intensity was about
2$\times$10$^{5}$ pps, with a purity of about 70% on target. The recoiled
protons were measured at the center-of-mass scattering angles of
$\theta_{c.m.}$$\approx$175.2${^\circ}$, 152.2${^\circ}$, and 150.5${^\circ}$
by three sets of $\Delta E$-$E$ telescopes, respectively. The excitation
function was obtained with the thick-target method over energies
$E_x$($^{22}$Mg)=5.5--9.2 MeV. In total, 23 states above the proton-threshold
in $^{22}$Mg were observed, and their resonant parameters were determined via
an $R$-matrix analysis of the excitation functions. We have made several new
$J^{\pi}$ assignments and confirmed some tentative assignments made in previous
work. The thermonuclear $^{18}$Ne($\alpha$,$p$)$^{21}$Na rate has been
recalculated based on our recommended spin-parity assignments. The
astrophysical impact of our new rate has been investigated through one-zone
postprocessing x-ray burst calculations. We find that the
$^{18}$Ne($\alpha$,$p$)$^{21}$Na rate significantly affects the peak nuclear
energy generation rate, reaction fluxes, as well as the onset temperature of
this breakout reaction in these astrophysical phenomena.
