Why do Integral Schemes have Normalizations?

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I know that affine integral schemes $\text{Spec} R$ have normalizations- namely the spectrum of the integral closure of $R$ in its field of fraction. However, how do we know normalizations exist for a general integral scheme $X$? Probably we would like to take a cover of $X$ by affine opens $U_{\alpha}$ and then glue their normalizations $\widetilde{U_{\alpha}}$. But how should we glue them?