Thevenin Voltage and Resistance Problem
The first thing you need to know before proceeding with the Thevenin's equivalent circuit is that the Vth that you find is not the voltage across your resistor (R2 in this case). In fact it is the voltage that appears at the voltmeter when your probes are at nodes A and B and WHEN YOU HAVE NOTHING CONNECTED TO THE TERMINALS A AND B.

Having said this I start with finding the Thevenin's resistance(Rth). For finding Rth, you need to short the voltage sources. Once you short the voltage sources, you end up with something like this circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

Now placing the ohmmeter in the terminals A and B gives you the resistance of 1.386k (5.6k || 2.5k || 7k). This is the required value of Rth.

Now in order to find the Thevenin's voltage, you can short one voltage at a time and carry out the calculation to find the voltage across the terminals AB due to each source individually. Then you can add up the results. I will short the  7V source and find the voltage at AB due to 15V source. 







simulate this circuit

Reducing the parallel combination of 2.5k and 7k (2k is not taken because it is open circuited actually) into single resistor, the equivalent resistance is 1.84k. Simply applying the voltage divider rule gives you the voltage of 3.71V across AB. This is the voltage at AB due to 15V.

Similarly analyzing the circuit by shorting 15V, you will get the voltage of 1.38V across AB due to 7V source. 

Adding these two voltages gives you the total voltage of 5.09V at AB which is the required value of Vth.

So the final circuit looks something like this:







simulate this circuit

If you place your 2k resistor at the terminal AB you will get the result same as your simulation.