How do I... Convert larger binary number in decimal

First File.cs
namespace ConsoleApplication1
{
    public class ConvertBinaryToHex    // this code is a procedure, i need it in objects
    {
        public String _BinToDec(int binaryNumber)
        {
            int decimalNumber = 0;
            int power = 0;


            if (binaryNumber == 0000)  // do you know how can I make just one return?(Below there is a return) and not 2. I mean this decision but without use return.
            {
                return "0";  
            }
            else
            {


                while (binaryNumber != 0)
                {
                    int aDecimalDigit;
                    aDecimalDigit = (binaryNumber % 10);

                    int aDecimalRisesToPower;
                    aDecimalRisesToPower = (int)Math.Pow(2, power);

                    int risesADecimalToAConsecutivePower = 0;
                    risesADecimalToAConsecutivePower = aDecimalDigit * aDecimalRisesToPower;

                    decimalNumber = decimalNumber + risesADecimalToAConsecutivePower;
                    binaryNumber = binaryNumber / 10;

                    power++;
                }

                String conversion = "  ";
                int theResidue;

                while (decimalNumber != 0)
                {
                    theResidue = decimalNumber % 16;
                    decimalNumber = decimalNumber / 16;

                    if (theResidue == 10)
                        conversion += "A";
                    else if (theResidue == 11)
                        conversion += "B";
                    else if (theResidue == 12)
                        conversion += "C";
                    else if (theResidue == 13)
                        conversion += "D";
                    else if (theResidue == 14)
                        conversion += "E";
                    else if (theResidue == 15)
                        conversion += "F";
                    else
                        conversion += theResidue;
                }

                // Para voltear el string
                char[] inputarray = conversion.ToCharArray();
                Array.Reverse(inputarray);
                return new string(inputarray); // this return will be ok, the first you have to change the logic. just this return in the code
            }
        }
    }
}
Program.cs
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            ConvertBinaryToHex   object1 = new ConvertBinaryToHex();

            //send this binary number as a parameter 

            //110101111101111000011010
            int a ;
            Console.WriteLine("please write 10 digit binary number : \a");
            // Convert.ToInt64(bno);
            a=Convert.ToInt32(Console.ReadLine());
            Console.WriteLine(object1._BinToDec(a));



            
        }
    }
}