The occurrence of Rosh Chodesh Tevet on Shabbat presents a rare scenario in which we take three Torah scrolls from the Aron (Ark) for the Torah reading. Since this day is Shabbat, Rosh Chodesh and Chanukah, we must read the Parashat Ha'shavua (weekly Torah portion), the section about Rosh Chodesh, and the appropriate reading for the given day of Chanukah. What is the proper procedure for this complicated reading? In which sequence do we conduct these three readings, how are the Aliyot distributed, when do we recite Kaddish, and which Haftara do we read?
The principle of "Tadir Ve'she'eno Tadir, Tadir Kodem" establishes that we have two Mitzvot before us, we afford precedence to the more frequent Mitzvahh. Therefore, we first read the weekly Torah portion, and we divide it into six Aliyot (as opposed to normal Shabbatot, when we divide the weekly portion into seven Aliyot). We then take the second Torah scroll and read for the seventh Aliya the section for Rosh Chodesh, which occurs more frequently than Chanukah. We begin with the verse "U've'yom Ha'Shabbat" and continue straight until the end of the Rosh Chodesh section.
Kaddish is recited after the conclusion of the Rosh Chodesh reading. Since Shabbat requires a minimum of seven Aliyot, the day's Torah reading is considered completed after the seventh Aliya, thus warranting the recitation of Kaddish.
After Kaddish we read for Maftir the appropriate section for that day of Chanukah, from the third Torah scroll. Kaddish is then repeated after the Maftir reading.
If seven Aliyot were called during the reading of the weekly Torah portion, then Kaddish is recited after that reading, as well, such that three Kaddishim would be recited in the context of the Torah reading in this situation.
For the Haftara, we read the selection of "Roni Ve'simchi" (from the Book of Zecharya), the standard Haftara for the Shabbat during Chanukah, which speaks of the Menorah. We read this Haftara, rather than the Haftara for Shabbat Rosh Chodesh, because the Haftara should correspond to the Maftir reading. Since for Maftir we read on this Shabbat the reading for Chanukah, we choose the Haftara that relates to Chanukah. Furthermore, this Haftara speaks about the Menorah, and we therefore read this Haftara in the interest of Pirsumei Nisa (publicizing the miracle of Chanukah).
Summary: When Rosh Chodesh Tevet occurs on Shabbat, the weekly Torah portion is divided into six Aliyot and read from the first scroll, followed by the Rosh Chodesh reading from the second scroll, which comprises the seventh Aliya. Kaddish is then recited, followed by the Maftir reading in the third scroll, for which we read the appropriate section for the given day of Chanukah. Kaddish is then repeated, followed by the Haftara for Shabbat Chanukah. If seven Aliyot were called for the weekly Torah portion, Kaddish is recited after that reading, as well.