  The $^{18}$Ne($\alpha,p$)$^{21}$Na reaction plays a significant role in
Type-I X-ray bursts. It is a major path in the breakout from the hot-CNO cycles
to the synthesis of heavier elements in the $\alpha p$-- and $rp$-processes. An
experiment to determine the cross section of this reaction was performed with
the ANASEN active-target detector system, determining the cross section at
energies between 2.5 and 4 MeV in the center-of-mass frame. The measured cross
sections for reactions populating the ground state in $^{21}$Na are consistent
with results obtained from the time-inverse reaction, but significantly lower
than the previously published experimental data of direct measurements. The
total cross sections are also compared with those derived from indirect methods
and statistical-model calculations. This experiment establishes a new
experimental data set on the excitation function of the
$^{18}$Ne($\alpha,p$)$^{21}$Na reaction, revealing the significance of the
excited states' contributions to the total reaction cross section and allowing
to separate the contribution of the $(\alpha,2p)$ reaction. The impact of the
measured cross section on thermal reaction rates is discussed.
