alpha<-5
beta<-6
theta<-seq(0, 100, length.out=101)
prior <- dgamma(theta, shape=alpha, scale=beta)
x<-42
posterior <- dgamma(theta, shape=x+alpha, scale=1/(1+1/beta) )

plot(theta, posterior, xlab=expression(theta), ylab="density", type="l")
lines(theta, prior, lty=3)

postdraw <- rgamma(2000, shape=x+alpha, scale=1/(1+1/beta))
r1<-hist(postdraw, freq=F, breaks=20, plot=F)
lines(r1, lty=3, freq=F, col="gray90")