Targ. Neophyti Gen 14:18: “And Melchizedek, the king of Jerusalem, who was the great Shem…”
St. Jerome, Hebrew Questions on Genesis, 14:18-19: "And Melchisedech king of Salem... Because our little book is, in a word, a collection of Hebrew questions or traditions, let us therefore introduce what the Hebrews think about this. They declare that this man is Sem, the son of Noah, and by calculating the years of his life, they show that he lived up to the time of Isaac; and they say that all the first-born sons of Noah were priests before Aaron performed the priestly office. Next, by 'king of Salem' is meant king of Jerusalem, which was formerly called Salem."
St. Ephraem, Commentary on Genesis 11:2: “This Melchisedek is Shem, who became a king due to his greatness; he was the head of fourteen nations. In addition, he was a priest. He received this from Noah, his father, through the rights of succession. Shem lived not only to the time of Abraham, as Scripture says, but even to [the time of] Jacob and Esau, the grandsons of Abraham. It was to him that Rebekah went to ask and was told, 'Two nations are in your womb and the older shall be a servant to the younger' (Gen 25,22-23). Rebekah would not have bypassed her husband, who had been delivered at the high place, or her father-in-law, to whom revelations of the divinity came continually, and gone straight to ask Melchizedek unless she had learned of his greatness from Abraham or Abraham’s son.
Because the length of Melchizedek’s life extended to the time of Jacob and Esau, it has been stated, with much probability, that he was Shem. His father Noah was dwelling in the east and Melchizedek was dwelling between two tribes, that is, between the sons of Ham and his own sons. Melchizedek was like a partition between the two, for he was afraid that the sons of Ham would turn his own sons to idolatry.”
For a further discussion see, James Kugel, The Bible As It Was (Cambridge: The Belknap Press of Harvard University Press, 1997), 160-161; John W. Bowker, The Targums and Rabbinic Literature. An Introduction to Jewish Interpretations of Scripture (Cambridge: Cambridge University Press, 1969), 196-199; Scott Hahn, Kinship By Covenant: A Biblical Theological Study of Covenant Types and Texts in the Old and New Testaments (Ann Arbor: University Microfilms, 1995), 171-81.