  We extract the total width of the top quark, Gamma_t, from the partial decay
width Gamma(t -> W b) measured using the t-channel cross section for single top
quark production and from the branching fraction B(t -> W b) measured in ttbar
events using up to 2.3 fb^-1 of integrated luminosity collected by the D0
Collaboration at the Tevatron ppbar Collider. The result is Gamma_t = 1.99
+0.69 -0.55 GeV, which translates to a top-quark lifetime of tau_t = (3.3 +1.3
-0.9) x 10^-25 s. Assuming a high mass fourth generation b' quark and unitarity
of the four-generation quark-mixing matrix, we set the first upper limit on
|Vtb'| < 0.63 at 95% C.L.
